At Lentech, the Data Scientist role sits at the intersection of mission-critical engineering and advanced analytics. You will be instrumental in supporting the Secure the Enterprise initiative, shifting manual system security evaluations toward automated, data-driven frameworks. Your work directly influences how federal and defense clients assess risk, monitor network data, and maintain compliance across complex lifecycles.
The role is far from purely theoretical. You will design and maintain ETL pipelines, build predictive models to identify outliers, and transform raw security logs into actionable intelligence. Whether you are working on simulation models or optimizing system performance, your contributions ensure that Lentech continues to deliver high-impact, technology-enabled solutions for its most sensitive government partners. You will be expected to thrive in a collaborative environment that values technical rigor, entrepreneurial spirit, and a deep commitment to mission success.
Initial Screens
reportedData Scientist covers at least four different jobs: experimentation, product analytics, causal work on observational data, and applied modelling that ships into a system. A screening call is the cheapest place to find out which of them is being hired for, and doing that diagnosis openly reads as senior rather than fussy. Ask what the last few pieces of work on the team actually were, and roughly how a week splits between querying, modelling and stakeholder time. Then say which parts of that you have done and which you have not. Claiming the whole range is the fastest way to be caught one round later.
What to demonstrate
- Whether you can distinguish the flavours of the role and locate your own experience inside one of them honestly
- Whether you name what you have not done instead of stretching to cover every line of the posting
- Whether your hard constraints (notice period, location, work authorisation, level) surface now rather than at offer stage
How to prepare
- Map the last two years of your time into rough percentages across query writing, experiment design, modelling and stakeholder work, so a question about scope has a real answer
- Mark every responsibility in the posting as done, adjacent or new, and prepare one sentence for each adjacent item naming the closest thing you have actually built
- Decide which logistics are non-negotiable before the call so you can state them in one sentence rather than negotiating live
Technical Evaluations
reportedBefore anything else, this round is a reading test. You are given a small schema and a question phrased in business language, and most of the difficulty sits in the gap between them. Who counts as an active user, does a refunded order still count as an order, is that date column an event time or a load time. Weak answers start typing immediately and compute something precise about the wrong population. Strong ones pin the definition in one sentence, name the column that encodes it, then write the query. On a timed assessment with nobody to tell, write the definition in a comment anyway.
What to demonstrate
- Whether an ambiguous term becomes a specific column and filter before any computation happens
- Whether you read the schema for keys and cardinality rather than only for column names
- Whether the result answers the question at the grain it was asked at, per user or per session or per day
How to prepare
- Take three metrics you already use and write down the exact filter and exact grain behind each, then practise stating one of them in a single sentence out loud
- On a schema you have never seen, spend the first minute writing what one row of each table means and which key it is unique on, then predict which joins can duplicate rows
- Rehearse a version where the definition changes halfway through, and edit the query you have instead of starting over
Behavioral Interviews
reportedRounds of this kind usually include one question about work that did not go well, and it is the part that carries the most information. Anyone can narrate a shipped win. What the interviewer learns from a project that stalled is how you behave without a result to hide behind: whether you noticed the problem yourself, how long it took, and who you told. Answers that route the failure onto a data pipeline or a reorganisation close the topic without answering it, and the follow-up comes back to your own part.
What to demonstrate
- Whether you found the error yourself or someone else found it, and how long it sat before anyone knew
- What you changed afterwards, stated as a check you now run rather than a lesson you now believe
- Whether the mistake you choose has real cost attached, such as a quarter of misdirected roadmap or a metric that was reported upward, instead of one that flatters you
How to prepare
- Choose a failure you caught yourself and be ready to say what tipped you off. A story where someone else caught it is still usable, but you will be asked why you missed it.
- Write down the check you added afterwards and where it lives now, so the correction is a concrete artefact rather than a resolution.
- Rehearse saying the cost out loud. Candidates shrink the number by instinct once the interviewer is in the room.
PracHub editorial advice for the preparation topics above.
Randomising an experiment at the user level when users share an account
Two problems fire at once. Colleagues in one workspace see each other's work and talk to each other, so a treated user changes the behaviour of a control user in the same account, which violates the no-interference assumption and biases the estimate toward zero. Separately, outcomes within an account are strongly correlated, so the effective sample size is roughly n / (1 + (m - 1) * rho) for m users per account and intra-class correlation rho, not n. With rho around 0.3 and twenty users per account that is a design effect near 6.7, meaning a user-level confidence interval is about two and a half times narrower than it should be and results cross significance thresholds on noise alone. Randomise the account and cluster the standard errors.
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.
Ignoring interference between units in a marketplace experiment
Ask whether one unit's treatment can change another unit's outcome through shared inventory, a matching pool, a social graph or a common budget. Where it can, randomise at a level that contains the spillover, such as region or time slice, and say explicitly what that costs you in statistical power.
Building features from data that postdates the prediction time
Check every feature against the timestamp at which the model would actually score, and drop anything computed from a window that includes or follows the label event. For a forecasting use case, split train and test by time rather than at random, and split by entity when the same entity recurs.
Choose a category, try a prompt, then open its approach, worked solution or follow-up when you need it.
Suggest a set of metrics to measure the efficacy of a new predictive m…
Suggest a set of metrics to measure the efficacy of a new predictive model for resource allocation.
Approach
- Frame the prediction: the label, the moment of prediction, and the action it triggers.
- Check what information would not exist at prediction time, and exclude it.
- Say how the offline result would be validated online before it is trusted.
Follow-up
- What would you monitor after launch to know the model is still valid?
- Where could label leakage enter this setup?
Audit daily usage rows for grain and arithmetic violations
You are handed fct_usage_daily as a pandas DataFrame with account_id, workspace_id, sku_code, usage_date, billable_quantity, included_quantity_applied, overage_quantity, list_amount_cents, discount_amount_cents, net_amount_cents, cogs_cents, is_restated, first_written_at and restated_at. The declared grain is one row per (account_id, workspace_id, sku_code, usage_date). Write audit(df) returning a DataFrame with one row per failing check: check name, failing row count, and one example key. Cover at minimum grain duplication, negative quantities or amounts, the identity net = list - discount, billable = included + overage, and rows where is_restated is true but restated_at is null.
Approach
- Check the grain before anything else with df.duplicated(subset=key, keep=False), and count rows rather than groups so a key appearing twice contributes 2 — if the grain is broken every arithmetic count below it is uninterpretable.
- Express each invariant as a boolean Series over the whole frame. The cent columns are integers and compare exactly, so use !=; the numeric(18,6) quantity columns need np.isclose with atol=1e-6 because included + overage is a decimal sum.
- Handle null as its own failure mode. Comparisons against NaN return False, so a check written as rows_that_pass = (a == b - c) silently files every null-amount row wherever the negation happens to land; build each check as violations = ~condition | column.isna().
- Collect the checks as a list of (name, mask) pairs and assemble the output in one pass, so adding a check is one line and every check reports in the same shape.
- Order the output with structural failures (grain, null keys) above arithmetic failures, and report zero-count checks too — a check that silently disappears when it passes is indistinguishable from a check that was never run.
Follow-up
- Which of these should block a dashboard refresh and which should only warn?
- Rows with is_restated = true legitimately change value after first write. How do you make yesterday's audit result reproducible?
- How would you extend this to catch a partition that is missing entirely rather than wrong?
Implement seven-day activation rate from its written definition
Given dim_account (account_id, created_at, is_internal, is_current) and fct_api_request (account_id, request_at, http_status, api_key_id, traffic_class), implement activation_rate(accounts, requests, week_start). Definition: the numerator is accounts whose first request with http_status < 400, api_key_id not null and traffic_class != 'synthetic_monitor' occurs no later than 168 hours after created_at; the denominator is non-internal accounts created during the ISO week starting week_start. All timestamps are tz-aware UTC. Return the rate and both counts, and refuse to report a week until every account in it has had its full 168 hours.
Approach
- Reduce dim_account to one row per account_id before joining anything. It is a type 2 dimension, so several versions of the same account exist; joining the versioned table to requests multiplies the denominator by the number of plan changes an account happened to make.
- Build the denominator first and freeze it: is_internal == False and week_start <= created_at < week_start + 7 days. Everything after this is a filter on the numerator only, because an account that never sent a request must still sit in the bottom of the fraction.
- Filter requests to qualifying rows and only then take groupby('account_id').request_at.min(). The first qualifying request is not the same object as the global first request filtered afterwards, and the two answers differ for every account whose first call was a 4xx.
- Left-join the first qualifying timestamp onto the cohort and test (first_ok - created_at) <= Timedelta(hours=168). NaT propagates to False in that comparison, which is the behaviour you want, but assert it rather than assume it.
- Guard reportability explicitly: if week_start + 7 days + 168 hours exceeds the maximum request_at in the data, the week is censored and will read as a drop, so return None or raise rather than emit a number.
Worked solution 20 min
- cohort = accounts.loc[~accounts.is_internal & accounts.created_at.between(week_start, week_start + pd.Timedelta(days=7), inclusive='left'), ['account_id','created_at']].drop_duplicates('account_id')
- ok = requests[(requests.http_status < 400) & requests.api_key_id.notna() & (requests.traffic_class != 'synthetic_monitor')]
- first_ok = ok.groupby('account_id', as_index=False).request_at.min(); m = cohort.merge(first_ok, on='account_id', how='left'); assert len(m) == len(cohort)
- activated = (m.request_at - m.created_at) <= pd.Timedelta(hours=168); return {'rate': float(activated.mean()), 'numerator': int(activated.sum()), 'denominator': len(m)}
Follow-up
- Median time-to-first-call is more informative. What breaks if you take the median over activated accounts only, and what estimator fixes it?
- How would you decide whether 168 hours is the right window rather than 72 or 336?
- An account signs up, does nothing for 20 days, then integrates heavily. Where does it land in this metric, and is that what you want?
How would you handle missing values or anomalies when parsing large-sc…
How would you handle missing values or anomalies when parsing large-scale network sensor data?
Approach
- Check whether any join is one-to-many before aggregating, or the sums inflate.
- Say which table is the grain you start from, and join outward from it.
- State the window function and its partition and ordering out loud before writing it.
Follow-up
- What breaks if events arrive late or out of order?
- How would you verify this result without re-running the same query?
Write a query using SQL window functions to calculate the moving avera…
Write a query using SQL window functions to calculate the moving average of system error logs over the last 30 days.
Approach
- State the window function and its partition and ordering out loud before writing it.
- Say which table is the grain you start from, and join outward from it.
- Check whether any join is one-to-many before aggregating, or the sums inflate.
Follow-up
- What breaks if events arrive late or out of order?
- How does the query change if the join becomes one-to-many?
Describe your process for optimizing a query that joins multiple high-…
Describe your process for optimizing a query that joins multiple high-volume compliance databases.
Approach
- Check whether any join is one-to-many before aggregating, or the sums inflate.
- State the window function and its partition and ordering out loud before writing it.
- Say which table is the grain you start from, and join outward from it.
Follow-up
- How does the query change if the join becomes one-to-many?
- What breaks if events arrive late or out of order?
Net revenue retention on a cohort frozen twelve months back
fct_subscription_period carries subscription_period_id, account_id, arr_cents, plan_code, term_start_date, term_end_date, booked_at, amendment_type, superseded_by_id (the subscription_period_id of the version that replaced this one, null on the live version of a lineage) and is_current. Compute net revenue retention for month M: the summed arr_cents at M for the set of accounts holding arr_cents > 0 at M-12, divided by that same set's arr_cents at M-12. An account can hold more than one live subscription, a churned account contributes zero rather than dropping out, and nothing signed after M-12 may enter either side. Return the ratio plus the expansion, contraction and churn components in cents.
Approach
- Write one reusable as-of ARR snapshot parameterised by a date: rows whose term brackets the date AND whose booked_at is at or before the date, then only the version of each lineage that is still live at that date, then sum arr_cents per account. The booked_at guard matters because an amendment signed in advance otherwise co-exists with the term it replaces and double counts the account.
- Collapse the lineage on superseded_by_id, not on any attribute of the contract. Keep a row when superseded_by_id IS NULL, or when the successor it points at was booked after the date. Ranking with ROW_NUMBER() OVER (PARTITION BY account_id, plan_code ...) instead is wrong in both directions: an amendment that moves the account from one plan_code to another puts the old and new versions in different partitions, so both are rank 1, both bracket the date, and the account's arr_cents is counted twice; and two genuinely concurrent subscriptions that happen to share a plan_code land in one partition, so one of them is deleted.
- Resolve the successor with a LEFT JOIN back to fct_subscription_period on subscription_period_id, and treat a missing successor as not superseded. An inner join would silently delete an account's ARR on a dangling pointer, which is a data-quality bug in the source, not a retention movement.
- Never use is_current for the M-12 side. is_current describes today; using it at the historical snapshot backdates the present contract onto last year's cohort and makes retention look like 100 percent by construction.
- Freeze the cohort from the M-12 snapshot where arr_cents > 0, then LEFT JOIN the M snapshot onto it and COALESCE the missing side to zero. An inner join deletes exactly the churned accounts, which is the single largest way this number gets overstated.
- Return a ratio of sums, not a mean of per-account ratios. The two are different estimands: contraction is floored at zero while expansion is unbounded, so the mean of ratios is both biased relative to the aggregate and far noisier on a skewed revenue base.
- Decompose per account on the delta: positive delta is expansion, negative delta with a non-zero M value is contraction, a zero M value is churn. The three components must reconcile to numerator minus denominator.
- Prove no leakage: any account whose first contract began after M-12 must be absent from both sides, and the cohort row count must be identical in the numerator and denominator.
Worked solution 45 min
- Write arr_asof(d) as a CTE or lateral: from fct_subscription_period s take rows with term_start_date <= d AND term_end_date >= d AND booked_at <= d, LEFT JOIN fct_subscription_period succ ON succ.subscription_period_id = s.superseded_by_id, keep the row when s.superseded_by_id IS NULL OR succ.subscription_period_id IS NULL OR succ.booked_at > d, then sum arr_cents per account_id across every surviving version.
- Sanity-check the lineage rule on one amended account before going further: at a date after the amendment, the account must contribute exactly one version per lineage even when the amendment changed plan_code, term dates or both.
- Materialise base = arr_asof(M-12) filtered to arr_cents > 0, and curr = arr_asof(M).
- LEFT JOIN curr onto base on account_id and COALESCE(curr.arr_cents, 0) AS arr_now.
- Compute nrr = sum(arr_now)::numeric / NULLIF(sum(base.arr_cents), 0), and the three components with SUM(...) FILTER on the sign of arr_now - base.arr_cents and on arr_now = 0.
- Reconcile: assert sum(arr_now) - sum(base.arr_cents) = expansion - contraction - churn, and assert the cohort account count is identical on both sides.
Follow-up
- Net revenue retention can rise while the business shrinks. Show one mechanism and name the guardrail that catches it.
- How do you handle an account that co-terms two subscriptions into one mid-window, so the subscription count changes but the money does not?
- Finance computes this from invoiced amounts and gets a different number. Which is right for which question?
How do you define "success" for a system that transitions from manual …
How do you define "success" for a system that transitions from manual to automated monitoring?
Approach
- State what result would change your recommendation, so the answer is falsifiable.
- Fix the population and the time window before naming any metric.
- Decompose the metric into the rates that drive it, and say which one you would check first.
Follow-up
- What would you do if the primary metric and the guardrail moved in opposite directions?
- Which segment would you cut first, and what would that rule out?
How would you design a dashboard to track the health of a new automate…
How would you design a dashboard to track the health of a new automated security authorization process?
Approach
- Name one primary metric, then the guardrail that stops it being gamed.
- Decompose the metric into the rates that drive it, and say which one you would check first.
- State what result would change your recommendation, so the answer is falsifiable.
Follow-up
- Which segment would you cut first, and what would that rule out?
- What would you do if the primary metric and the guardrail moved in opposite directions?
If a key system security metric suddenly drops by 10%, how would you i…
If a key system security metric suddenly drops by 10%, how would you investigate the root cause?
Approach
- Name one primary metric, then the guardrail that stops it being gamed.
- Fix the population and the time window before naming any metric.
- Restate the decision this analysis has to support, and who acts on the answer.
Follow-up
- Which segment would you cut first, and what would that rule out?
- How would you detect that the metric is being gamed rather than genuinely improving?
How do you determine the required sample size for an A/B test on a sec…
How do you determine the required sample size for an A/B test on a security tool’s performance?
Approach
- Decide the analysis before seeing data, including how long it runs and when you look.
- Name the randomisation unit first; it decides the variance and what the test can detect.
- State the primary metric and the minimum effect worth shipping, then size the test.
Follow-up
- What would you do if you could not randomise at all?
- How would you handle interference between treated and control units?
What are the most common experimentation pitfalls you have encountered…
What are the most common experimentation pitfalls you have encountered when testing new system features?
Approach
- State the primary metric and the minimum effect worth shipping, then size the test.
- Name the guardrails that would stop a launch even on a positive primary result.
- Name the randomisation unit first; it decides the variance and what the test can detect.
Follow-up
- What would you conclude if the result is positive but the test is underpowered?
- How would you handle interference between treated and control units?
Measure named coverage at an assignment threshold you cannot randomise
Named customer-success coverage is assigned to accounts whose arr_cents at fiscal-year start is at least $50,000. About 88% of accounts above the line receive a named owner and about 9% below it do, through documented exceptions. Roughly 200 accounts sit within $15,000 of the cutoff, and gross logo retention on the renewal-eligible base runs near 0.88. Estimate the effect of coverage on renewal, name the design, state its identifying assumptions and the test for each, and say whether this data can answer the question at all.
Approach
- Say first why the naive comparison is unusable, because the answer turns on it. Coverage is assigned to the largest accounts because they are valuable and to distressed accounts because they are at risk, so selection runs in both directions at once and the naive estimate can come out with either sign depending on which rule dominated. Matching on observed size does not fix it: the risk signal that triggered coverage is the same signal that predicts renewal.
- Name the design and the estimand together. This is a fuzzy regression discontinuity with the $50,000 threshold as an instrument for coverage. The estimator is the Wald ratio, the jump in renewal at the cutoff divided by the jump in coverage probability of 0.88 - 0.09 = 0.79, and the estimand is a local average treatment effect for accounts near $50,000 whose coverage is actually decided by the rule, not the average effect of coverage across the book.
- Fit it as local linear regression on each side with a triangular kernel and a data-driven bandwidth (Imbens-Kalyanaraman, or Calonico-Cattaneo-Titiunik with bias-corrected robust intervals). A global polynomial fitted across the whole ARR range is not a substitute and is well documented to manufacture spurious jumps at interior points.
- Test the assumptions instead of asserting them. Run a density test for bunching just above $50,000, which is exactly what a sales team structuring a deal at $50,001 to win coverage would produce and which invalidates the design outright. Check continuity at the cutoff of employee_band, acquisition_channel, deployment_model and pre-period usage. Run placebo cutoffs at other ARR values, and plot the estimate against bandwidth.
- Do the power arithmetic before promising an answer. With about 100 accounts per side and a renewal base rate of 0.88 (standard deviation 0.325), the sharp-design MDE at 80% power and two-sided 0.05 is 2.80 * 0.325 * sqrt(2/100) = about 12.9 percentage points, and dividing by the 0.79 first stage gives about 16 points on the fuzzy estimand. No plausible effect of assigning an account manager is that large.
- Give the honest verdict with a route forward attached. This design cannot answer the question on one fiscal year. Pool several years at the same cutoff, accepting the added assumption that the coverage model and the threshold's meaning were stable across them, or ask for a randomised holdout among accounts near the threshold, which is cheap to justify precisely because the rule is already arbitrary there.
Worked solution 45 min
- Plot the density of arr_cents in bins around $50,000 and run the manipulation test; stop and report the design as invalid if bunching is present.
- Plot the first stage, share of accounts with a named owner against ARR, and confirm the jump is close to 0.79 and locally flat on each side.
- Fit local linear regressions of renewal on ARR either side of the cutoff with a triangular kernel and data-driven bandwidth, take the Wald ratio, and report bias-corrected robust intervals.
- Run covariate-continuity checks, placebo cutoffs, and a bandwidth-sensitivity curve for the point estimate.
- Compute the MDE at the realised effective sample and write the verdict, powered or not, with the alternative design named.
Follow-up
- The density test shows clear bunching just above $50,000. Is any part of the design salvageable, and what would you report instead?
- Coverage also correlates with support ticket severity. Does adding severity as a control help, hurt, or do nothing in a regression discontinuity?
- If you pool five fiscal years to gain power, what new identifying assumption have you taken on, and how would you check it?
Consumption per account always dips in the last four days
Your consumption dashboard reads billable units per paying account from fct_usage_daily (account_id, sku_code, usage_date, billable_quantity, net_amount_cents, is_restated, first_written_at, restated_at, updated_at). Every refresh shows the final three to five days declining, and a review is scheduled on that shape. Establish empirically how long a usage_date takes to settle, separately per sku_code, and specify the trailing exclusion window every reported figure should use. Derive the number from the data; do not adopt a convention.
Approach
- Treat settlement as a measurable curve rather than a belief: for each historical usage_date, compare the total captured as of age d days against that date's final settled total.
- Do it per sku_code, because metering paths differ. A per-request SKU lands within hours, while a storage-month SKU is produced by a daily sweep and lands later, so one global lag number is wrong for at least one of them.
- Pick the age at which a stated percentile of dates reaches a stated completeness threshold, for example the tenth percentile date reaching 99.5 percent of final, and take the slowest SKU's age as the dashboard's exclusion window.
- Separate late arrival from restatement. Late rows raise totals and are fixed by waiting; restatements can move either way and are not, so report the share of the gap from each.
- Encode the exclusion inside the query rather than in the chart, so anyone reusing the SQL inherits the rule, and never compare a fresh partial period against a settled one.
Follow-up
- Month-end invoicing needs a number before settlement completes. How would you publish an early estimate with an honest uncertainty band attached?
- What monitor would tell you the settling time has changed, without anyone remembering to re-run this analysis?
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 ↗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 ↗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 ↗Practice prompt ↗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 ↗Worked solution ↗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 ↗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 ↗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 ↗Practice prompt ↗Worked solution ↗Expand any day for tasks and deliverables. Your progress is saved on this device.
Most of the questions in this section reduce to one thing: can you be handed a vague request and come back with something useful? Prepare an example where the ask was underspecified, you chose an interpretation, and you said out loud which interpretation you chose. Describing how you narrowed the question matters more than the technique you eventually used.
Describe a situation where you had to lead a project with significant …
Describe a situation where you had to lead a project with significant technical ambiguity.
Approach
- Name the disagreement or constraint, and how you resolved it with evidence.
- Quantify the outcome, including what you would not claim credit for.
- Close with what you would do differently, concretely.
Follow-up
- What would you do differently if you ran that project again?
- What did you decide not to do, and why?
State the measured impact of your own work honestly
You are asked for the business impact of a renewal-risk worklist you shipped nine months ago. Customer success used it, and renewals in the covered segment came in 6 points above the prior year. Coverage was assigned by the team itself: they worked the top of your list and also the accounts they were already worried about. Produce the impact claim you are willing to defend, the number you refuse to claim, and the design you would have asked for at the start.
Approach
- The interviewer is probing whether you can separate what you shipped from what you caused, and whether you would have built the measurement in rather than reconstructing it afterwards. Both halves are being scored.
- Name the confound precisely. Coverage assignment is doubly selected: the largest accounts get an owner because they are valuable, and distressed accounts get one because they are at risk. The naive covered versus uncovered comparison mixes a strong positive selection with a strong negative one and can come out with either sign depending on which rule dominated. Matching on account size does not fix it, because the risk signal that triggered coverage is the same signal that predicts the outcome.
- Split the claims by what each needs to be true. Ranking quality is defensible from precision at k on out-of-time renewals. Adoption is defensible from timestamps showing what share of listed accounts were contacted. The outcome claim is not defensible without a design, and saying so is the point of the exercise.
- Look for identification before giving up on it. A capacity cut-off, a territory boundary, or a period in which the list existed but was unstaffed can assign coverage for reasons unrelated to account health, and any of those supports a bounded estimate.
- State the design you would ask for now and its price: a randomly withheld slice of the list, held for two renewal quarters, with the expected cost in renewals stated openly. That cost is what it takes to be able to answer this question at all.
- Give a bounded number rather than none. Six points with an explicit statement of how much of it you can attribute is more useful than either claiming the whole figure or declining to quantify anything.
Follow-up
- Your manager wants the 6 points in a promotion packet. What wording do you accept, and what do you strike?
- What would have had to be true for the naive covered versus uncovered comparison to be valid?
- If the holdout costs the team real renewals, how do you justify asking for it, and to whom?
Defend a churn number twelve times the one in the board deck
You recompute logo churn on the renewal-eligible base from fct_subscription_period, counting only accounts whose term_end_date fell in the month and allowing a 45-day grace for late paperwork. Annualised, about 14 percent of accounts that reach a renewal date do not renew. A revenue leader has been quoting 1.2 percent to the board for two quarters, computed by dividing non-renewed accounts by the entire customer base in each month and printing that monthly figure with no period attached. You have 20 minutes with that leader and the finance lead. Decide which number is reported from now on, and what happens to the two quarters already published.
Approach
- The interviewer is probing whether you can hold a correct definition under social pressure without turning it into a competence dispute. Open by reproducing their 1.2 percent exactly, with their denominator and their months, so the disagreement is arithmetic both sides can see rather than a claim about who was careless.
- Separate the two defects, because they are different in kind. The denominator is wrong: on annual contracts only about one twelfth of the base reaches a renewal date in any month, so an account eleven months from renewal sits in the denominator while being structurally incapable of entering the numerator, which suppresses the rate by a factor near twelve. The period is merely unstated: a monthly figure printed beside annual revenue targets gets read as an annual rate.
- Say out loud that those two defects nearly cancel in the level, before the leader finds it. Twelve times 1.2 percent is about 14 percent, which is your number. That is the strongest thing you can say in the room, because it proves both figures rest on the same non-renewal count and moves the meeting onto which denominator and which period get published rather than onto whose query is right.
- The level is recoverable; the series is not. Non-renewals in a month are the eligible base for that month times the churn rate, so dividing by a fixed whole base makes the published line proportional to how many contracts happen to come up that month. Where signings cluster at quarter ends, the eligible base in a quarter-end month can be several times a quiet month's, and the month-over-month moves the board has been reading as satisfaction are the signing calendar.
- Separate the measurement change from a business change. Nothing got worse this week; the loss rate was always this. Bring net revenue retention over the same period as a ratio of sums on a cohort frozen twelve months earlier, because logo churn concentrated in small accounts can sit beside healthy revenue retention, and that combination is the actual story.
- Offer a migration path rather than a correction. Report both rates for one quarter with a written bridge, restate the prior two quarters in an appendix instead of silently, and pin the definition, including the period it is stated over, somewhere finance and product both read it. Concede the limits of your own number: the 45-day grace means the most recent 45 days are not reportable, and churn must be dated on term_end_date rather than on updated_at. A strong answer volunteers this; a generic one only defends.
Follow-up
- The leader multiplies their monthly figure by twelve, lands on your annual number, and concludes nothing was ever wrong. What do you say?
- The leader says publishing the corrected rate costs the team its credibility with the board this quarter. What do you do?
- Gross logo retention worsened while net revenue retention improved. Which do you lead with, and what does the combination tell you about who is leaving?
- 01
Describe a situation where you had to lead a project with significant technical ambiguity.
- 02
You are asked for the business impact of a renewal-risk worklist you shipped nine months ago. Customer success used it, and renewals in the covered segment came in 6 points above the prior year. Coverage was assigned by the team itself: they worked the top of your list and also the accounts they were already worried about. Produce the impact claim you are willing to defend, the number you refuse to claim, and the design you would have asked for at the start.
- 03
You recompute logo churn on the renewal-eligible base from fct_subscription_period, counting only accounts whose term_end_date fell in the month and allowing a 45-day grace for late paperwork. Annualised, about 14 percent of accounts that reach a renewal date do not renew. A revenue leader has been quoting 1.2 percent to the board for two quarters, computed by dividing non-renewed accounts by the entire customer base in each month and printing that monthly figure with no period attached. You have 20 minutes with that leader and the finance lead. Decide which number is reported from now on, and what happens to the two quarters already published.
Is this an official Lentech interview guide?
No. It is PracHub's own research and practice material for the Data Scientist role at Lentech. Rounds and questions reflect what candidates have reported, not a process Lentech has published, and they change over time. Confirm the current format and scope with your recruiter.
PracHub interview research ↗How much preparation time is recommended?
Given the technical nature of the role, plan for at least 2–3 weeks of focused practice on SQL window functions, statistical testing, and reviewing your own past projects for behavioral deep dives.
PracHub interview research ↗What differentiates successful candidates?
Candidates who stand out are those who can connect their technical skills directly to the Lentech mission. Don't just explain how you used a model; explain how that model improved security or saved time for the client.
PracHub interview research ↗Is this role fully remote?
Due to the nature of the security clearances and the specific mission requirements in locations like Fort Meade and Linthicum, most roles require on-site presence.
PracHub interview research ↗Sources & methodology 3 sources ↗
No official company page is cited. Rounds and questions come from candidate reports and PracHub editorial material; each source shows the date it was read.
- 01PracHub interview research ↗
PracHub editorial research into this company and role, maintained with this guide. Candidate-reported, not an employer publication.
PracHub page · Accessed 2026-09-22 - 02PracHub Data Scientist practice ↗
Cross-company practice questions for this role.
PracHub page · Accessed 2026-09-22 - 03PracHub interview preparation framework ↗
The framework the preparation plan follows.
PracHub page · Accessed 2026-09-22
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