What makes the analysis hard here is the shape of the data rather than its volume. The account population is small by consumer standards, often a few thousand paying accounts, and revenue across it is extremely skewed, so the top fraction of a percent holds a large share of the total and any mean over accounts is effectively a statement about a handful of customers. Outcomes within an account are strongly correlated because colleagues share workspaces and influence each other, which both inflates apparent sample size and lets treatment leak between arms when randomisation is done at the user level. Contracts are annual, so churn is a rare, censored, calendar-driven event that most monthly metrics are structurally incapable of measuring. And the telemetry itself is largely machine-generated: a customer changing one line in a CI configuration can multiply request volume overnight without a single human deciding anything about the product.
The skills that carry the most weight are, in order, SQL across mismatched grains with an explicitly defended denominator, cohort and survival thinking applied to contracts rather than to individuals, and experiment design that survives clustering and heavy tails. Concretely that means as-of joins against a versioned subscription table, randomisation at the account level with the design effect accounted for in the power calculation, pre-registered winsorisation or a capped metric when the outcome is revenue, and variance reduction using pre-period usage as a covariate. Unit economics matters more than in most domains because infrastructure cost is a real and allocable per-account quantity, so being able to say which accounts are unprofitable and why is a routine expectation. The last skill is unglamorous and decisive: when the product number and the finance number disagree, being able to say which one is right, by what definition, and what the reconciling difference is.
Data science work in business software, infrastructure and developer tooling sits between a telemetry stream and a contract system, and most of the job is making those two agree. The unit of analysis is the account, not the person: an organisation signs, an organisation renews, and a hundred engineers inside it are correlated observations rather than a hundred independent ones. The recurring questions are narrow and concrete, covering which accounts will expand at renewal and which will contract, what a unit of usage costs to serve for a given account at its negotiated discount, whether a packaging change moved consumption or merely moved it between SKUs, and which accounts a capacity-limited customer-success team should contact this week. The deliverable is usually an account-level list with a threshold attached, handed to a team with finite capacity, so a ranking without an operating point is an unfinished piece of work. The audience is rarely another data scientist, so a definition also has to survive being repeated by someone who did not write it.
Account-grain SQL against versioned contracts
editorialQuery usage, membership and contract tables together at the account grain without fanning out rows, and resolve every contract question as of a date rather than as of now. This is the substrate for nearly every other analysis in the domain, because the subscription table is versioned and the usage tables are not.
What to demonstrate
- Writing an as-of join from a daily usage fact to fct_subscription_period, selecting the version whose term_start_date and term_end_date bracket the usage_date, rather than joining to is_current and silently backdating today's plan over last year's usage
- Joining a one-row-per-account dimension to a many-row-per-account fact without multiplying revenue, including the case where dim_account is type 2 and contributes several versions in the window
- Stating the denominator and its exclusions out loud before writing the SELECT, specifically is_internal accounts, non-production environments, service accounts and synthetic traffic classes
- Recognising when a question needs a rollup from fct_api_request and when it must come from fct_usage_daily, and explaining why the two will not reconcile exactly
How to prepare
- Write one query returning monthly net revenue, allocated COGS and gross margin per account, joined as-of to the contract version live during each month, and confirm the total ties to the sum of net_amount_cents
- Practise the type 2 dimension pattern until it is automatic: filter to the version live at the event timestamp, never to is_current, and be able to say what breaks when you get it backwards
- Build a query that separates interactive, CI, batch and synthetic traffic into columns for the same account and week, and look at how different the engagement story is under each filter
- Rehearse explaining, in two sentences, why request counts from fct_api_request and billable quantities from fct_usage_daily diverge, including retries, failed requests and restatement
Retention, expansion and contract cohort arithmetic
editorialBuild net revenue retention, gross logo retention and expansion rate from a versioned contract table, and defend each denominator. This is the numerical language the domain uses to talk about itself, and small definitional errors here change the headline number by tens of points.
What to demonstrate
- Constructing net revenue retention as a ratio of sums over a cohort frozen twelve months earlier, and explaining why the mean of per-account ratios gives a different and much noisier answer given that contraction is floored at zero while expansion is not
- Restricting logo churn to the renewal-eligible base and articulating why a monthly rate on the whole base is off by roughly the reciprocal of the annual renewal fraction
- Handling censoring properly when contracts have not yet reached their renewal date, including when a survival estimator is required instead of a simple rate
- Separating booked_at from term_start_date and knowing which one belongs in a sales-effectiveness question and which in a revenue question
How to prepare
- Compute net revenue retention two ways on the same data, ratio of sums and mean of ratios, and be ready to explain the gap from the shape of the expansion distribution
- Derive from first principles how much a naive monthly churn rate understates the truth when all contracts are annual, and be able to do that arithmetic aloud
- Work through the ways net revenue retention can rise while the business shrinks, and name the guardrail that catches each one
- Build a renewal-eligible cohort for a given month from fct_subscription_period, including the 45-day grace for late paperwork, and sanity-check that roughly a twelfth of the base appears
Experimentation with clustered, skewed outcomes
editorialDesign and read out experiments where the randomisation unit is an account, the sample is a few thousand clusters at most, and the outcome of interest is revenue or consumption with an extreme right tail. Most standard experimentation instincts are calibrated on large-sample consumer data and mislead here.
What to demonstrate
- Choosing the account as the randomisation unit and computing power with the design effect 1 + (m - 1) * rho, rather than quoting a user-level sample size
- Recognising when the experiment is simply not powerable on revenue and proposing a proximate outcome, a longer horizon, or a switchback design for a platform-level change that cannot be randomised across accounts at all
- Applying variance reduction with a pre-period covariate, for example CUPED using the account's prior consumption, and stating the approximate variance reduction as one minus the squared correlation
- Pre-registering the winsorisation or capping rule and being explicit that it changes the estimand rather than pretending it is a neutral cleaning step
How to prepare
- Run a power calculation for an account-randomised test on consumption, with and without the design effect, and note how much longer the clustered version has to run
- Practise the diagnostic for a heavy tail: plot the running sample variance against sample size and say what it means when it does not stabilise
- Prepare one clear explanation of interference inside a shared workspace and how it biases a user-level estimate toward zero
- Have a switchback or stepped-wedge design ready for an infrastructure change that is global by nature, including how you would handle carryover between periods
Usage telemetry hygiene and unit economics
editorialTurn raw machine-generated traffic into a defensible measure of human value, and attach cost to it. The gap between what the platform serves and what a customer values is wider in this domain than almost anywhere else, and allocating infrastructure cost to an account is a routine expectation rather than a finance task.
What to demonstrate
- Systematically excluding internal accounts, non-production environments, synthetic monitors, load tests, service-account traffic and retried requests, and being able to quantify how much each exclusion removes
- Computing gross margin at the account level from net_amount_cents and cogs_cents, and identifying structurally margin-negative accounts rather than reporting a blended rate that conceals them
- Establishing the metering settling time empirically from first_written_at against restated_at, and excluding an appropriate trailing window from every reported figure
- Explaining how a rising error rate can inflate metered volume through client retries, and designing a metric that does not reward that
How to prepare
- Take one week of request-level data and produce the same engagement number under four progressively stricter traffic filters, then be ready to argue for one of them
- Build the per-account margin distribution and answer, with a number, what share of revenue sits in margin-negative accounts
- Measure how much a given usage_date's total shifts between first write and settlement, and set a lag window from that measurement rather than from convention
- Write down the specific mechanism by which a platform outage can increase billable units, and the guardrail metric that would expose it
Renewal-risk and expansion scoring with an operating point
editorialBuild an account-level model that ranks renewal risk or expansion opportunity, with leakage-free features and a threshold tied to the capacity of the team that will act on it. The output is a worklist, so ranking quality at the top matters far more than calibration across the whole distribution.
What to demonstrate
- Constructing features strictly from data available before the prediction point, and spotting the leakage that comes from features updated retroactively, such as a churn reason code, a downgrade amendment or a support ticket opened after the renewal conversation began
- Choosing an evaluation metric that matches how the list is used, meaning precision or recall at the k the team can actually work, not a global AUC over thousands of accounts most of which will never be contacted
- Handling class imbalance and small positive counts honestly, since annual contracts produce few churn events per period, and knowing when the honest answer is that there is not enough signal to model
- Turning the score into a decision by attaching a threshold to the expected value of an intervention and the capacity constraint, and stating what happens to accounts below the line
How to prepare
- Build a leakage audit habit: for every feature, name the timestamp that guarantees it existed before the prediction date, and drop anything you cannot date
- Practise reporting precision at k for several plausible team capacities instead of a single headline number
- Prepare an argument for why a simple, inspectable model that a customer-success team will actually trust can outperform a stronger model they ignore
- Work out how you would evaluate whether the intervention itself worked, given that assignment of coverage is not random, and name the design you would ask for
1 candidate reports. Individual accounts describe a particular role and hiring cycle.
ZScalar Account Executive interview: mock cold-email exercise
I had a recruiter call and, about a week later, entered a structured process that moved quickly. It began with a hiring-manager conversation and a mock cold-email exercise. That first stage showed they were testing both how I thought about the role and how I would communicate in a realistic sales motion. If I had advanced, the later steps would have included another account-executive interview, p…
Read full experiencePracHub editorial advice for the preparation topics above.
Comparing accounts that received a sales or customer-success touch against those that did not
Assignment of coverage is deliberate and pulls in both directions at once: the largest accounts get a named owner because they are valuable, and the accounts showing distress get one because they are at risk. The comparison therefore mixes a strong positive selection with a strong negative one, and the naive estimate can come out with either sign depending on which assignment rule dominated during the period examined. Nothing about matching on observed size fixes this, because the risk signal that triggered coverage is usually the same signal that predicts the outcome. It needs either an actual randomised or staggered rollout of coverage, or a design built on a capacity constraint or territory boundary that assigns coverage for reasons unrelated to account health.
Computing monthly churn against the entire customer base when contracts are annual
An annual contract has no opportunity to churn except at its renewal date, so an account that is eleven months from renewal is in the denominator while being incapable of appearing in the numerator. The resulting rate is smaller than the real one by roughly the ratio of the base to the renewal-eligible base, and it oscillates with the seasonality of when deals were originally signed rather than with anything about the customers. The corresponding trap on the other side is counting a churn on the date the record was updated rather than on term_end_date, which shifts losses into whichever month the operations team did its paperwork.
SQL that silently fans out on a one-to-many join
State the grain of each table and the grain you want in the result before writing the join. Pre-aggregate the many side to the join key, or use EXISTS or a window function, and verify with a row count against COUNT(DISTINCT id) rather than trusting that the numbers look plausible.
Reaching for a model before the target metric exists
Before naming an algorithm, write down the label, the prediction time, and the action that changes when the score crosses a threshold. If you cannot say what decision the output drives, any modelling choice is guesswork dressed up as method.
Choose a category, try a prompt, then open its approach, worked solution or follow-up when you need it.
Permutation-test a consumption experiment randomised at account level
An experiment randomised 900 accounts into two arms. You have one row per account: account_id, arm, consumption_28d (billable units after launch) and consumption_pre (the 28 days before). Consumption is heavy-tailed and the largest account is several percent of the total. Write a permutation test from scratch: winsorise at the pooled 99th percentile as a pre-registered rule, use the difference in arm means of the winsorised outcome as the statistic, and obtain a two-sided p-value from 20,000 relabellings of the account-level arm vector. Report the observed effect, the p-value, and the same test on a CUPED-adjusted outcome.
Approach
- Be precise about what the permutation test needs. Under the sharp null of no effect for any account, the outcomes are exchangeable across arm labels, and the test is valid for ANY statistic T(outcomes, labels) provided the identical function is applied to the observed labels and to all 20,000 relabellings. The pooled 99th percentile is a function of the outcome vector alone, so recomputing it inside the loop returns the same number 20,000 times: that is wasted CPU, not a bias, and hoisting it out is an optimisation rather than a correctness fix. Say plainly that capping at all changes the estimand from mean consumption to mean capped consumption; it is not a neutral cleaning step.
- The mistake that does invalidate the test is an asymmetry between the observed statistic and the permuted ones, and the easiest way to create it is to derive the cleaning rule from the observed arm labels and then freeze it — winsorise each arm at its own observed 99th percentile, hold those two caps fixed, and permute. The observed value is then computed with caps matched to its own partition while every relabelling is scored with caps belonging to a different one, so the null distribution no longer answers the question the p-value claims to answer. A per-arm cap recomputed consistently inside every permutation is a valid test, but it estimates a contrast whose two sides are capped at different thresholds, so prefer the pooled cap on estimand grounds and pre-register it.
- Permute the account-level arm vector, because the account is the randomisation unit. Relabelling anything finer — users, workspaces, requests — generates a null distribution narrower than the design actually supports and returns p-values that are anti-conservative.
- Vectorise the null: tile the treatment indicator into a (B, n) matrix and permute along axis 1 with rng.permuted(..., out=...). The statistic is a difference of means, so the treated sum alone determines it and the whole null is one matrix-vector product. Use the two-sided p-value (1 + count(|stat_perm| >= |stat_obs|)) / (B + 1); the plus-one on each side is not cosmetic, it keeps the p-value away from exactly zero and keeps the test valid at finite B.
- For CUPED, fit theta = cov(y, x) / var(x) on the pooled data and use that same theta for the observed statistic and every relabelling. Pooled theta, like the pooled cap, carries no label information, so where in the loop you compute it is again only a performance question; fitting theta within arms is what goes wrong, because the adjusted outcome then depends on the labels and an observed-label fit frozen across all 20,000 relabellings breaks the match between observed and permuted statistics. x must be measured entirely before launch, which consumption_pre is. Expected variance reduction is about 1 - corr(y, x)^2; measure the achieved reduction from the two null distributions rather than asserting it.
Worked solution 45 min
- cap_y = np.quantile(df.consumption_28d, 0.99); y = np.minimum(df.consumption_28d.to_numpy(float), cap_y); cap_x = np.quantile(df.consumption_pre, 0.99); x = np.minimum(df.consumption_pre.to_numpy(float), cap_x)
- t = (df.arm == 'treatment').to_numpy(); n1 = int(t.sum()); n0 = len(t) - n1; obs = y[t].mean() - y[~t].mean()
- rng = np.random.default_rng(11); L = np.tile(t.astype(np.int8), (20_000, 1)); rng.permuted(L, axis=1, out=L); s1 = L @ y; stats = s1/n1 - (y.sum() - s1)/n0
- p = (1 + int(np.sum(np.abs(stats) >= abs(obs)))) / (20_000 + 1)
- theta = np.cov(y, x, ddof=1)[0,1] / np.var(x, ddof=1); y_adj = y - theta*(x - x.mean()); repeat steps 2 to 4 on y_adj and compare stats.std(ddof=1) between the two runs.
Follow-up
- The p-value is 0.04 with the cap and 0.31 without it. What do you report, and what did you pre-register?
- Colleagues in a shared workspace can see the treated behaviour. How does that change the design and the estimate?
- How many accounts would you need to detect a 5% lift given this outcome's distribution?
Quantify billable volume created by retries after server errors
From fct_api_request (request_id, account_id, endpoint, idempotency_key, is_retry, http_status, request_at, billable_units), measure how much billable volume in a 28-day window is retry traffic that followed a 5xx. Group requests into attempt chains by (account_id, endpoint, idempotency_key) ordered by request_at; a request is error-driven if any earlier attempt in its chain returned 5xx. Requests with a null idempotency_key cannot be chained, so report them as their own class rather than assuming each is unique. Return billable_units split into first-attempt, error-driven retry, other retry and unchainable, per account.
Approach
- Split the population before measuring anything. A null idempotency_key is not a chain of one, it is an unknown; report its share of billable_units first, because if it is 40% of volume then the headline estimate is a lower bound and the deliverable has to say so.
- Within chainable rows, sort by (account_id, endpoint, idempotency_key, request_at) and derive 'any earlier attempt failed' with arithmetic rather than a per-group lambda: with is5 = (http_status >= 500), the per-chain cumsum minus the row's own value is positive exactly when an earlier attempt in that chain returned 5xx. A groupby-apply gives the same answer and is unusable at five million rows.
- Do not take is_retry as the definition. It is set by the client whenever an idempotency_key is resent, which covers retries after client-side timeouts and after 4xx as well; compute the flag yourself and then cross-tabulate it against is_retry, because the disagreement is a finding in its own right.
- Aggregate billable_units by (account_id, class) and assert the classes sum to each account's total. The spine sets billable_units to zero on 5xx responses, so the failed attempt contributes nothing and the whole inflation sits in the successful retry that follows it.
- Report the per-account share and look at its distribution, not the fleet total. One account in a retry storm dominates any blended figure, which is the same failure that makes a fleet-wide error rate useless.
Follow-up
- An account's error-driven share is 22%. Is that the platform's fault or the client's, and what do you look at next?
- How would you define a consumption-based north-star metric that an outage cannot inflate?
- Chains straddle the 28-day boundary. How large is that bias and in which direction?
Join daily usage to the contract version live that day
fct_usage_daily has account_id, usage_date and net_amount_cents. fct_subscription_period has account_id, subscription_period_id, plan_tier, term_start_date, term_end_date, arr_cents and is_current, with one row per contract term version and every amendment inserting a new row. Attach to each usage row the subscription_period_id whose term brackets usage_date (term_start_date <= usage_date <= term_end_date). pd.merge_asof and an interval-condition merge are unavailable; use sorting and numpy.searchsorted. Then report monthly net revenue by plan_tier. Terms for one account do not overlap, and usage may fall outside every term.
Approach
- Say out loud why the cheap version is wrong: joining on is_current stamps today's plan tier onto last year's usage, so every account that upgraded has its history reclassified and revenue-by-tier becomes a function of when the query ran.
- Sort each account's terms by term_start_date and use np.searchsorted(term_start, usage_date, side='right') - 1 to get the last term that started on or before the usage date. Do it per account group, or globally after encoding (account_id, date) into one monotone key.
- searchsorted only enforces the left edge. Validate the right edge afterwards — usage_date <= the candidate's term_end_date — and set the match to NA where it fails. That NA is usage in a gap between contracts and must stay visible instead of being folded into the expired term.
- Assert non-overlap before trusting the lookup, and write the assertion so it is capable of passing. prev_end = terms.groupby('account_id').term_end_date.shift() is NaT on each account's first row, and NaT < Timestamp evaluates to False rather than NA, so a comparison followed by .fillna(True) has nothing left to fill and the assertion fires on every account's first term whatever the data looks like. Guard the null yourself: assert (prev_end.isna() | (prev_end < terms.term_start_date)).all(). The failure mode of getting this wrong is not a false alarm you notice once — it is an assertion someone deletes because it never passes, after which overlapping terms make searchsorted return one of them with no trace in the output.
- Aggregate after the join, grouping by (usage_date month, plan_tier) with dropna=False so the unmatched bucket appears as its own row and the total still ties to the ungrouped sum of net_amount_cents.
Follow-up
- An amendment takes effect on the 17th of a month. How do you report that month's revenue by tier?
- What changes if terms can overlap because of a co-term amendment?
- How would you verify this against a SQL implementation using a BETWEEN condition?
Running commitment burn-down and the date consumption crosses it
fct_subscription_period gives committed_amount_cents, term_start_date and term_end_date for each account's current version where pricing_model = 'committed_consumption'. fct_usage_daily gives account_id, workspace_id, sku_code, usage_date and net_amount_cents, at one row per workspace and SKU per day. Inside each account's current term, return the running total of net_amount_cents by usage_date, the first usage_date on which that running total reaches committed_amount_cents, and the fraction of the term elapsed at that point. Accounts that have not reached their commitment must still appear, with a null crossing date.
Approach
- Collapse usage to one row per (account_id, usage_date) first. The fact is grained by workspace and SKU, so a raw running total leaves several rows per date and the first-crossing date becomes dependent on the arbitrary order of rows inside that day.
- Restrict to the term with usage_date BETWEEN term_start_date AND term_end_date on the account's current row, so no prior term's consumption leaks into this term's burn-down.
- Compute SUM(net_cents) OVER (PARTITION BY account_id ORDER BY usage_date ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW). Name ROWS explicitly: the default frame is RANGE, which includes all peer rows at the same ORDER BY value and would hand back the whole day's total on each of that day's rows.
- Extract the crossing with MIN(usage_date) FILTER (WHERE running_cents >= committed_amount_cents) grouped per account, which returns NULL for accounts still under commitment instead of dropping them.
- Elapsed fraction is (crossing_date - term_start_date)::numeric / NULLIF(term_end_date - term_start_date, 0); Postgres date subtraction yields whole days, so the guard matters for same-day terms.
- Exclude the trailing days still inside the metering settling window, measured from first_written_at against restated_at, and say how many days you cut and why.
Follow-up
- Turn this into an end-of-term overage forecast. What breaks if you extrapolate a linear run rate on a consumption product?
- An account crosses its commitment at 40 percent of the term. Is that an expansion signal or a billing-surprise risk, and what would you check to tell them apart?
Collapse retried requests into logical operations per account
fct_api_request carries request_id, account_id, api_key_id, idempotency_key (null when the caller supplied none), is_retry, request_at and http_status. For one ISO week, count logical operations rather than HTTP calls: requests sharing (account_id, api_key_id, idempotency_key) collapse to the earliest one, while every request with a null idempotency_key is its own operation. Return per account the raw request count, the logical operation count and the ratio between them, and state which direction a rising 5xx rate pushes that ratio.
Approach
- Split the stream before deduplicating. GROUP BY and PARTITION BY treat NULLs as equal to one another, while the = operator does not, so a single grouping pass folds every null-key request in an account into one 'operation' and can cut the count by orders of magnitude.
- Deduplicate the keyed rows with ROW_NUMBER() OVER (PARTITION BY account_id, api_key_id, idempotency_key ORDER BY request_at, request_id) and keep rn = 1. Including request_id in the ORDER BY makes the result deterministic when two rows share a millisecond, which they will.
- UNION ALL the null-key rows back in unchanged; they need no dedup and must not pass through the partition.
- Aggregate per account: count() over the raw week, count() over the union, and the ratio of the two. Report the ratio, not the difference, so accounts of very different sizes are comparable.
- Cross-check against is_retry, which flags a resend of the same idempotency key: raw minus logical should sit close to the count of is_retry rows, and a large gap means clients are resending without a key and the dedup is under-counting duplicates.
- Name the inversion explicitly: 5xx responses provoke client retries, so amplification rises exactly when reliability falls, and any engagement metric built on raw request counts will show growth during an outage.
Worked solution 25 min
- Filter to the ISO week with request_at >= :week_start AND request_at < :week_start + interval '7 days', and record the raw count per account as the baseline.
- Build keyed_dedup: SELECT ... ROW_NUMBER() OVER (PARTITION BY account_id, api_key_id, idempotency_key ORDER BY request_at, request_id) AS rn FROM the week WHERE idempotency_key IS NOT NULL, then keep rn = 1.
- Build unkeyed: the same week's rows WHERE idempotency_key IS NULL, taken as-is.
- UNION ALL the two, then GROUP BY account_id selecting the logical count; join back to the raw counts and compute raw::numeric / logical.
- Validate on one account by comparing raw - logical against COUNT(*) FILTER (WHERE is_retry) for that account and explaining any gap.
Follow-up
- Which of these two counts belongs in the billable-units metric, and which in an engagement metric?
- An account's amplification ratio jumps from 1.05 to 3.0 in a day. Name three causes and the query that separates them.
Pick the randomisation unit when workspaces and queues are shared
A collaboration feature was tested by randomising 18,000 users 50/50. dim_user_membership shows a median of 9 activated, non-service users per paying account, and weekly successful interactive requests per user carries an intra-class correlation of 0.25 within an account. The reported lift is 4% at p = 0.05. Separately, the same team wants to evaluate a change to the admission-control queue, which is shared by every account in a region. Give the correct randomisation unit for each test, the effective sample size and corrected p-value for the first, and the design for the second.
Approach
- Separate the two failures of user-level randomisation, because they need different fixes and only one of them is a variance problem. Interference: a treated colleague changes a control colleague's behaviour inside a shared workspace, which biases the estimate toward zero. Correlated outcomes: users inside an account are not independent draws, which understates the variance. Clustering the standard errors fixes the second and leaves the first entirely intact.
- Compute the design effect: 1 + (m - 1) * rho = 1 + 8 * 0.25 = 3.0. Effective sample is 18,000 / 3 = 6,000 users, corresponding to roughly 2,000 accounts. Standard errors are understated by sqrt(3) = 1.73, so a reported t of 1.96 is really 1.13 and the honest two-sided p is about 0.26, not 0.05.
- Re-run the first test randomised on account_id with inference clustered on account_id, or equivalently as a regression on account-level means. The account-means version is the conservative one and is usually easier to defend to a non-specialist audience, at the cost of some efficiency when cluster sizes are very unequal.
- For the queue change, account randomisation fails too, and for a different reason: treated and control accounts contend for the same finite capacity, so the control arm is mechanically affected by the treated arm and the contrast estimates a property of a mixed system rather than the effect of the policy. Randomise time instead, with a switchback alternating the admission policy at the region-by-30-minute-slot level.
- Specify the switchback so it is actually valid. Impose a washout at the start of each slot at least as long as the p99 queue drain time and discard requests enqueued before the switch; randomise slot order rather than strictly alternating, because a fixed alternation aliases with the hourly and weekday traffic cycle; block on hour-of-day so both policies see peak and trough; and cluster inference on the slot, which is the unit that was randomised.
- State the power consequence plainly: the cluster count is slots, not requests. Fourteen days of 30-minute slots gives 672 slots, and the variance to plan against is between-slot variance in the outcome, which is far larger than between-request variance and is what makes switchbacks expensive.
Follow-up
- The intra-class correlation was estimated at 0.25 from a 300-account pilot. If it is really 0.40 the design effect becomes 4.2. How do you plan under that uncertainty rather than betting on the point estimate?
- Under what conditions is user-level randomisation still the right choice even though users share accounts?
- The queue change is expected to leave a four-hour retry backlog. What does carryover of that length do to a 30-minute switchback, and what would you run instead?
Build an activation metric tree for a self-serve onboarding rebuild
The self-serve signup flow is being rebuilt. You have dim_account (account_id, created_at, is_internal, first_workspace_at, is_current) and fct_api_request (account_id, workspace_id, environment, api_key_id, http_status, traffic_class, request_at). Build a three-layer metric tree from signup through to production dependency, name the primary metric for this specific change, and state the fixed measurement window and the denominator for each layer. Two of the three layers share a denominator and the third does not, so say which invariant still holds across the tree once the denominators differ. Say which layer the onboarding team can actually move and which it cannot, and why raw signup counts are not on the tree.
Approach
- Fix the cohort once: accounts created in cohort period P with is_internal = false, collapsed to one row per account_id since dim_account is type 2. Say out loud that employee, demo and load-test accounts are removed, otherwise the tree partly measures internal testing.
- Layer one, qualified signup, on the cohort denominator: accounts recording at least one successful request (http_status < 400) with traffic_class in ('interactive','batch') within 14 days of created_at. This replaces raw signup counts, which measure registration-form friction and duplicate sign-ups from one organisation rather than demand.
- Layer two, activation, also on the cohort denominator: the share of the cohort whose first successful request with a non-null api_key_id arrives within a fixed 168 hours of created_at. Fixed window, not trailing, so that cohort weeks are directly comparable and no cohort is censored.
- Layer three, production depth, on a conditional denominator: the share of activated accounts holding a workspace with environment = 'production' whose trailing 7-day count of successful non-synthetic requests exceeds a floor calibrated from the data, for example the volume above which 90-day retention stops rising. A floor picked by intuition makes this a metric about the floor. Conditioning on activation is the right operational choice, because the question it answers is whether accounts that got started go on to depend on the product, but it means this rate is not on the same scale as the two above it: a conditional rate of 0.50 sits perfectly happily underneath a 0.30 activation rate. To place it back on the funnel, multiply it by the activation rate and report that product as the cohort share.
- Align the traffic_class filter across all three layers, otherwise the underlying account sets do not nest: an activation definition that admits traffic_class = 'ci' will count accounts that layer one excluded, and the tree stops being a funnel. With mixed denominators the invariant that survives is on account sets and therefore on counts, not on rates. Then name the primary metric for this change, the seven-day activation rate, because the onboarding flow controls the path to first successful call and nothing below it. Production depth is the lagging check that the activation gain was real rather than one sample request.
Follow-up
- Activation rises 6 points and the production-depth rate among activated accounts is flat 60 days later. Say what happened to the cohort share of production-depth accounts, and why a flat conditional rate is the better of the two readings available here.
- Median time-to-first-successful-call answers the same question with more information. Why is it a worse weekly dashboard metric, and what estimator would make it honest?
Define success for widening the free usage allowance
A proposal triples the free-tier included allowance on the requests_thousands SKU. Finance expects the count of weekly active organisations to rise. You have fct_usage_daily (billable_quantity, included_quantity_applied, overage_quantity, net_amount_cents, cogs_cents, is_restated), dim_account (account_status, is_internal, converted_to_paid_at) and fct_api_request (traffic_class, http_status, environment). Define the primary metric this change should be judged on, two guardrails, and the numeric decision rule you would agree before launch. State explicitly how the obvious metric can be satisfied without the product getting better for anyone.
Approach
- Name the mechanism before naming any metric. A larger allowance moves quantity out of overage_quantity into included_quantity_applied, so net_amount_cents falls while cogs_cents is unchanged, and accounts that previously sat under the production volume floor now cross it without paying anything. A count of active accounts therefore moves for reasons that have nothing to do with the product.
- Set the primary metric at the account grain: weekly active organisations with at least one successful request (http_status < 400) in a workspace where environment = 'production' and traffic_class in ('interactive','batch'), over accounts with is_internal = false. Report accounts that existed before launch separately from the post-launch signup cohort, so the number is not quietly an acquisition metric.
- Attach the two guardrails that close the gaming routes. First, gross margin per account, (sum(net_amount_cents) - sum(cogs_cents)) / sum(net_amount_cents) computed per account per month and reported as a distribution, specifically the count and revenue share of margin-negative accounts. Second, the free and trial share of weekly active organisations.
- Add the conversion link the change is supposed to buy: free-to-paid conversion within 90 days for the affected cohort, read from converted_to_paid_at. Without it, the primary metric rewards permanent free use indefinitely.
- Write the decision rule with numbers before launch: ship if weekly active organisations rise by at least the pre-agreed threshold, 90-day free-to-paid conversion does not fall, and the count of margin-negative accounts stays under a stated cap. Note that the margin guardrail can only be read after the metering settling window has closed, since fct_usage_daily rows restate in place.
Worked solution 20 min
- Compute per-account monthly gross margin from fct_usage_daily as (sum(net_amount_cents) - sum(cogs_cents)) / sum(net_amount_cents), grouped by account_id and calendar month, restricted to usage_date values older than the measured metering settling window.
- Simulate the allowance change on the trailing settled month: for each account, move min(overage_quantity, allowance_delta) from overage_quantity into included_quantity_applied and recompute net_amount_cents at that account's existing list and discount rates, leaving cogs_cents untouched.
- Recount weekly active organisations under the simulated state with the production volume floor applied, and split the delta into accounts that were already active and accounts newly crossing the floor.
- Report the before and after margin distribution, the count of accounts crossing into negative margin, and the revenue share those accounts hold.
Follow-up
- Weekly active organisations jump 14% in the first week after launch. How much of that can you attribute to the change with no control group, and what would you need to do better?
- An account settles at usage permanently just below the new allowance and never pays. Does your primary metric count it as a success, and should it?
Error rate halves while severe support tickets double
The fleet-wide customer-visible error rate fell from 1.8 percent to 0.9 percent, while sev1 and sev2 tickets and reopened_count rose across the same fortnight. Using fct_api_request (account_id, environment, sdk_name, traffic_class, http_status, request_at, billable_units), fct_usage_daily (account_id, sku_code, usage_date, billable_quantity) and fct_support_ticket (account_id, severity, opened_at, reopened_count, linked_incident_id), determine whether reliability improved, and if not, identify precisely which rows are missing and from when. Deliverable: a diagnosis backed by an independent corroborating source.
Approach
- Distrust an improvement that contradicts an independent operational signal. Two sources disagreeing is itself the finding; decide which one is more likely to be broken before explaining either.
- Recompute the rate as the metric tree defines it, per account first and then as the share of accounts above the reliability target. A single global average is dominated by whichever account sends the most traffic, so a fleet number can fall while a quarter of accounts get worse.
- Audit for missingness rather than for badness: count fct_api_request rows per hour split by environment, sdk_name and status class, indexed against the trailing same-hour baseline. A partial ingestion failure shows as a step drop confined to one slice, not a uniform decline.
- Reconcile against a source the request pipeline does not feed, such as implied request volume from fct_usage_daily for the same accounts and dates. If the usage table is flat while request rows fell, rows are missing rather than traffic.
- Test whether the missingness is differential by status, which is the mechanism that fakes an improvement: if 5xx rows are written on a path that stopped while 2xx rows were unaffected, the numerator falls faster than the denominator and the rate drops with nothing improving.
- Close with the affected window, the affected slice and a restated series marked unreliable across that window, rather than a silently patched number.
Follow-up
- The missing rows are unrecoverable. How do you present that fortnight in a series people compare week over week?
- What monitor would have caught this within an hour, and what is its false-positive cost on a normal quiet weekend?
For someone who has spent the last year in notebooks, dashboards or modelling work and has not written raw SQL under time pressure. The first four days rebuild query fluency against a fixture you control and can verify by hand; the last three attach that fluency to the rest of the loop.
Prepare, practise & reflect
One practical outcome each day. Spend longer where you need it.
0 / 7 done01Build a fixture you can check answers against
- Create a local Postgres or SQLite database with four tables (users, sessions, events, orders) holding roughly 200 rows you generated yourself, so you know the contents well enough to predict every result.
- Deliberately seed the cases that break queries: a user with no sessions, a session with no events, two orders sharing a timestamp, a NULL in one join key, and one duplicated user row.
- Before writing any SQL, hand-compute five answers on paper (how many users placed at least one order, median orders per ordering user, and three others) and save them as the ground truth for the week.
Deliverable: A one-command seed script plus a text file of five hand-computed answers to grade every later query against.
Practice prompt ↗Practice prompt ↗Worked solution ↗02Joins, filters and NULL semantics
- Answer "which users have no orders" three ways (LEFT JOIN with IS NULL, NOT EXISTS, NOT IN) and confirm that the NOT IN version returns zero rows once the subquery contains a NULL, because the comparison is never TRUE.
- Reproduce the LEFT JOIN that silently collapses to an inner join by putting a right-table predicate in WHERE, then fix it by moving the predicate into the ON clause, and record both row counts.
- Create a fan-out bug on purpose by joining orders to order_items and summing the order total, then correct it with a pre-aggregated subquery and explain in one line which table changed the grain.
Deliverable: One annotated .sql file holding the three join traps, each with the wrong result and the corrected result side by side.
Practice prompt ↗Practice prompt ↗03Window functions and frames
- Write three window queries against the fixture: a running order total per user, the rank of each order within its user by value, and the day gap to that user's previous order, then check each against the day-one ground truth.
- Run ROW_NUMBER, RANK and DENSE_RANK over a column containing ties, print all three side by side, and write one sentence on when each is the correct choice.
- Switch one query from the default frame (RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW, which is what you get when ORDER BY is present and no frame is written) to ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW, and explain why the output differs only when the ORDER BY column has duplicates.
Deliverable: Three verified window queries plus a short note explaining the RANGE versus ROWS difference in your own words.
Practice prompt ↗Practice prompt ↗Worked solution ↗04The four analytical query patterns
- Write a monthly retention grid: first order month per user, then months-since-first as the column, and verify that month zero equals the cohort size exactly.
- Sessionize the events table under a 30-minute inactivity rule using LAG plus a cumulative sum over a new-session flag.
- Build a four-step funnel that counts distinct users rather than events at each step, and state the rule you applied to a user who reaches step three without ever logging step two.
Deliverable: One file with the retention, sessionization and funnel patterns, each carrying a one-line note on the assumption it bakes in.
Practice prompt ↗Practice prompt ↗05Write SQL the way you will have to write it live
- Set a 12-minute timer and solve three medium prompts in a plain editor with no execution and no autocomplete, then run them and tally syntax errors separately from logic errors.
- Narrate one solution aloud while writing it, stating the grain of each intermediate result (one row per user, one row per user-day) before you type its body.
- Rewrite your slowest solution as a CTE chain where every CTE name states its grain, and time yourself re-solving it from blank.
Deliverable: A recording of one narrated solution plus an error tally that separates syntax from logic.
Practice prompt ↗Practice prompt ↗Worked solution ↗06One day for everything that is not SQL
- Write the preconditions of the two-sample t-test from memory, then check them: independent observations, and a difference in means whose sampling distribution is approximately normal, which at large sample sizes follows from the central limit theorem rather than from normality of the raw values.
- Write the difference between an odds ratio from logistic regression and a relative risk, and state the condition under which the two are close (low outcome prevalence).
- Prepare a 90-second answer to "how would you know this model is any good" that names the metric, the baseline you would beat, and the cost of the errors you care about.
Deliverable: One page of notes covering test preconditions, the odds-ratio caveat and the model-quality answer.
Practice prompt ↗07Full loop rehearsal
- Run a 45-minute mock with someone willing to interrupt: 20 minutes of SQL, 15 minutes defining a metric, 10 minutes on a past project.
- Re-solve from blank the two queries you were slowest on this week and compare the times against day five.
- Write a five-line answer to "walk me through a project" that puts a number in the first sentence and names the decision the work changed.
Deliverable: Mock feedback notes plus a timed project narrative you can deliver without reading it.
Practice prompt ↗Worked solution ↗Expand any day for tasks and deliverables. Your progress is saved on this device.
Sometimes the honest read is that the initiative did not work, and the person who commissioned the analysis was hoping otherwise. Interviewers want to know whether you softened it. Prepare the case where you delivered an unwelcome result, how you presented the uncertainty without hiding behind it, and what the team did next.
Disagree with a product manager about an adoption claim
A product manager is about to present that a new SDK release drove a 40 percent rise in requests among adopting accounts, computed from fct_api_request counts grouped by sdk_version. You find the rise is concentrated in traffic_class equal to ci, that rows with is_retry true grew alongside it, and that restricting to interactive non-retry traffic leaves a 3 percent lift. The launch review is in two days. Decide how you raise this, with whom and in what order, and what you propose the claim becomes.
Approach
- The interviewer is probing whether you can correct a colleague without ambushing them, and whether your own counter-analysis carries the caveats theirs lacked. Go to the product manager privately before the review. A correction delivered in the room is a status move and loses the argument you are actually trying to win.
- Bring a decomposition rather than a verdict: the same accounts and window, requests split by traffic_class with retries held out as their own column, so their 40 percent and your 3 percent reconcile line by line and neither has to be taken on trust.
- Reproduce their figure exactly first. If you cannot land on 40 percent with their method, you do not yet know what you are disagreeing with.
- Ask whether the continuous-integration lift is itself valuable. An account wiring the SDK into its pipeline has increased integration depth, which is the dominant switching cost in this domain, so the honest claim may be that integration depth rose while interactive usage moved 3 percent. Improving the claim beats deleting it.
- Name the mechanism that makes the raw count dangerous: clients retry when the platform degrades, so retry volume climbs exactly when the customer is most at risk. Pull the 5xx rate for the same accounts and window before anyone concludes anything, and note that billable_units is zero on 5xx rows, so request counts and billable quantities diverging is itself the signal.
- Close with a standing definition for launch metrics so the next release does not repeat the exercise.
Follow-up
- The product manager argues that continuous-integration traffic is real usage and declines to split it out. Is that position defensible, and under what metric definition?
- Suppose the 5xx rate for those same accounts also rose 40 percent. What is the claim now?
- The review happens and the raw number is presented regardless. What do you do next, and what do you not do?
Announce a metric fix that cuts the headline number
Weekly active organisations, the count on the company dashboard, has never excluded rows where dim_account.is_internal is true, and it counts traffic with traffic_class in synthetic_monitor and load_test. Correcting both reduces that count by 11 percent and removes most of the growth reported over two quarters. The figure appears in a board deck and in two teams' quarterly goals, one written on the count and one on the weekly active organisation ratio, whose denominator is accounts whose account_status was in ('trial','free','active_paid') through the week. Decide the order in which you tell people, what the dashboard shows during the transition, and what you propose happens to goals already set against the old definition.
Approach
- The interviewer is probing whether you can land a correction as an operational change with a plan attached, rather than as an announcement other people then have to clean up after.
- Quantify each exclusion separately before telling anyone: internal accounts, synthetic monitors, load tests. Three known quantities are a discussion; one alarming total is an argument.
- Be precise about which side of the metric each exclusion touches, because one team's goal is on a count and the other's is on a ratio. The traffic-class filters remove requests, so they shrink the numerator only. Dropping internal accounts removes them from the ratio's denominator as well, since internal accounts carry ordinary account_status values and therefore sit in that denominator. Internal accounts are active in almost every week while the real base is not, so the numerator loses a larger share than the denominator and the ratio falls by less than the count does. Compute both and say which one the 11 percent is before anybody assumes.
- Check whether the trend changes, not only the level. A constant 11 percent shift is a rebasing and nothing more. A shift that widens over time means the reported growth was partly internal or synthetic, which makes the existing goals unachievable as written and changes what you are asking teams to do.
- Sequence the disclosure: the metric owner and the two teams whose goals move first and privately, then the board channel with a written bridge, then the dashboard. The dashboard is last because a number that changes without explanation is read as instability rather than as a fix.
- Run both series for one reporting period with the bridge visible, restate history rather than letting the series break at a date, and set the date the old series is removed.
- Propose the goal treatment yourself: rebase each target by the shift measured on the metric that target is written against, rather than leaving each team to negotiate individually, which is where corrections of this kind usually die.
Follow-up
- One team's quarterly goal is now unreachable. Rebase the target or let it miss, and what does each choice teach the organisation?
- How would this have been caught when the metric was first defined?
- What else on that dashboard shares this failure mode, and how would you find out this week?
Scope an open-ended request to predict account churn
A customer success director asks for a list of accounts about to churn. You know only that the team has six people and that contracts are annual. Available data is fct_subscription_period, fct_usage_daily, fct_api_request, fct_support_ticket and dim_account. Before writing any code, produce the questions you need answered, a proposed definition of about to churn, and the shape of the artefact you would hand back, including the operating point that turns a score into a decision.
Approach
- The interviewer is probing whether you convert a vague request into a decision with a capacity constraint attached. A candidate who starts talking about model families has already failed the exercise.
- Pin the event and the horizon first. Churn is only possible at term_end_date, so the population is accounts renewing in the next 60 to 90 days, not the whole base. Ask explicitly whether contraction and downgrade count as churn or only full non-renewal, because the three have different base rates and different interventions.
- Pin the action and the capacity. Six people times a realistic number of meaningful interventions per week gives k, and k is what the list is ranked to. Evaluate on precision at k rather than a global AUC over accounts that will never be contacted.
- Audit leakage before choosing features. Every feature needs a timestamp proving it existed before the prediction date. A downgrade amendment, a churn reason code, and a ticket opened after the renewal conversation started are all leaks that will make the offline number look excellent and the live list useless.
- Ask for the counterfactual now rather than later. Coverage is assigned deliberately, so without a held-out slice agreed at the start the intervention can never be evaluated, and you will be asked for its impact in nine months regardless.
- Propose the smallest artefact that closes the loop: a weekly ranked list sized to capacity with two or three inspectable reasons per row, plus a stated policy for accounts below the line.
Follow-up
- The director insists all accounts are in scope, not only those renewing soon. How do you answer without simply refusing?
- Historical non-renewals number about 30 a year. At what point do you tell them a model is the wrong tool and a rules list is better?
- Which candidate features would you drop purely because you cannot date them?
- 01
A product manager is about to present that a new SDK release drove a 40 percent rise in requests among adopting accounts, computed from fct_api_request counts grouped by sdk_version. You find the rise is concentrated in traffic_class equal to ci, that rows with is_retry true grew alongside it, and that restricting to interactive non-retry traffic leaves a 3 percent lift. The launch review is in two days. Decide how you raise this, with whom and in what order, and what you propose the claim becomes.
- 02
Weekly active organisations, the count on the company dashboard, has never excluded rows where dim_account.is_internal is true, and it counts traffic with traffic_class in synthetic_monitor and load_test. Correcting both reduces that count by 11 percent and removes most of the growth reported over two quarters. The figure appears in a board deck and in two teams' quarterly goals, one written on the count and one on the weekly active organisation ratio, whose denominator is accounts whose account_status was in ('trial','free','active_paid') through the week. Decide the order in which you tell people, what the dashboard shows during the transition, and what you propose happens to goals already set against the old definition.
- 03
A customer success director asks for a list of accounts about to churn. You know only that the team has six people and that contracts are annual. Available data is fct_subscription_period, fct_usage_daily, fct_api_request, fct_support_ticket and dim_account. Before writing any code, produce the questions you need answered, a proposed definition of about to churn, and the shape of the artefact you would hand back, including the operating point that turns a score into a decision.
Are these confirmed ZScalar interview questions?
No. Every prompt here is an original PracHub practice exercise written for the Data Scientist role and for B2B software and infrastructure data problems. This guide does not claim to reproduce ZScalar's interview questions, rounds or hiring timeline. Confirm the actual format, team and scope with your recruiter.
PracHub Data Scientist practice ↗How do I structure an answer to "define a metric for this"?
Name the decision the metric supports, then specify it: numerator, denominator, unit of analysis (user, session or event), time window, and who is excluded. Then give one way it can be gamed or move for the wrong reason, and a guardrail that would catch that. Close with how you would validate it against an event whose effect you already know. Vagueness about the denominator is the most common way this answer falls apart.
PracHub Data Scientist practice ↗What if I think the interviewer's premise is wrong?
Test it before announcing it. Ask a clarifying question that separates the two readings, because the premise is often a deliberate simplification. If it really is wrong, say what you observed, why it changes the answer, and offer to proceed under either reading. Some cases plant a flawed premise on purpose and noticing it is the question being asked. Being right about it and graceless about it still costs you.
PracHub Data Scientist practice ↗What should I do when I genuinely cannot answer a question?
Say what you do know, name the gap precisely, and propose how you would close it. "I have not used that test, but I would reach for a permutation test here because I cannot justify normality at this sample size" is a strong answer. Inventing a confident wrong answer or going quiet both score badly. Interviewers calibrate on how you behave at the edge of your knowledge, because that is where most real work happens.
PracHub Data Scientist practice ↗Sources & methodology 2 sources ↗
Official role evidence, timestamped platform data and clearly labeled preparation advice.
- 01PracHub Data Scientist practice ↗
Cross-company practice questions for this role. Not an employer question record.
platform · Accessed 2026-09-22 - 02PracHub interview preparation framework ↗
The shared preparation framework these checkpoints and the seven-day plan follow.
platform · Accessed 2026-09-22
Explore related engineering environments and compare the preparation focus.
Zscaler Data Scientist Interview Questions & Guide 2026
Data-quality drills for Zscaler data scientist prep: survivorship and selection bias, leakage through time-ordered features, and extreme values where provenance decides the call. A logging fault or unit slip gets corrected or dropped; a gen
Zeta Data Scientist Interview Questions & Guide 2026
Data-quality drills for Zeta data scientist prep: survivorship and selection bias, leakage through time-ordered features, and extreme values where provenance decides the call. A logging fault or unit slip gets corrected or dropped; a genuin
Zebra Technologies Data Scientist Interview Questions & Guide 2026
Causal-inference exercises for Zebra Technologies data scientist prep: difference-in-differences, matching, instrumental variables, and knowing when observational data simply cannot answer the question asked.
Zapier Data Scientist Interview Questions & Guide 2026
Communication practice for Zapier data scientist candidates: turn an analysis into a recommendation, name the uncertainty honestly, and answer the follow-up a skeptical stakeholder will raise.