A Data Scientist at Nextera Energy Services plays a pivotal role in driving the clean energy transition. As one of the largest producers of wind and solar energy in the world, the company relies heavily on data-driven decision-making to optimize energy generation, manage grid reliability, and deliver innovative energy solutions to millions of customers. In this role, you will work at the intersection of advanced analytics, machine learning, and physical energy infrastructure, transforming massive datasets into actionable strategic insights.
The impact of this position is felt across the entire business ecosystem. Whether you are building predictive maintenance models for wind turbines, forecasting energy demand to stabilize the grid, or segmenting customer data to optimize retail energy plans, your algorithms will directly influence operational efficiency and sustainability efforts. It is a highly collaborative environment where data scientists partner with software engineers, energy traders, and product managers to scale models into production.
What makes this role exceptionally compelling is the sheer complexity and scale of the data. You will be working with high-frequency time-series data from smart grids, meteorological inputs, and customer usage patterns. To succeed, you must not only possess deep technical expertise but also have a passion for solving real-world energy challenges that have a direct environmental and economic footprint.
Automated Assessment
reportedA handful of shapes account for most of what gets asked in this format: a ranking or deduplication inside groups, a running or rolling total, a period-over-period comparison, and a cohort tracked forward over time. Recognising the shape quickly is most of the speed here; deriving it from scratch while a clock runs is where the time goes. Know that a window function keeps every row while a GROUP BY collapses them, and know which one the question needs. If the exercise is in Python instead of SQL, the same shapes arrive as groupby with transform, shift and merge, and the same grain mistakes are available.
What to demonstrate
- Whether you reach the right construct without a detour, such as ROW_NUMBER over a partition to deduplicate instead of a self-join against a MAX subquery
- Whether you know what your window frame actually is, since adding ORDER BY inside OVER changes the default frame and silently changes a running total
- Whether the thing runs. A near-miss that throws an error scores below a plainer query that returns the right rows.
How to prepare
- Write each of the four shapes once from memory against a small schema and keep the working version somewhere you will reread it: dedupe with ROW_NUMBER, a running total, a month-over-month change with LAG, and a retention table
- Compute one running total twice on data with tied timestamps, once on the default frame and once with ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW, and look at where the two disagree
- If Python is on the table, rebuild the dedupe and the running total with groupby and cumsum, then assert the two implementations return identical rows
Recruiter Phone Screen
reportedA screening call is a matching exercise run by someone who will not evaluate your statistics. They are checking that the work described on your resume is work you personally did, and that its scope matches the level the role is written for. Logistics get settled in the same half hour so nobody spends an interviewer's afternoon on a mismatch. The answer that fails is the one narrated in the plural. If every sentence is 'we built' and 'the team decided', there is nothing specific to write down about you. Name the piece that was yours, the decision you made inside it, and what changed after.
What to demonstrate
- Whether the ownership implied by your resume survives one round of follow-up about who actually did which part
- Whether your described scope (data size, stakeholders, what shipped) matches the seniority the role is written at
- Whether timeline, location and compensation expectations make the rest of the loop worth scheduling
How to prepare
- Rewrite your top three resume bullets in the first person singular, each with the decision you made and what moved afterwards, then say them out loud once so the 'we' does not return under pressure
- Attach one number to each project: the baseline, the change, and the window it was measured over. Where impact was never measured, say that plainly rather than inventing a figure
- Settle your compensation range before the call and give it as a range with a reason behind it, such as current total comp or a competing timeline, instead of deflecting the question twice
Technical Discussions
reportedThis round decides whether someone can hand you a schema and a question and trust the number that comes back. Correctness under a clock is the bar, not clever syntax. The habit that separates strong from weak answers is checking the grain: after every join, know how many rows you expect and whether the count moved. Most wrong answers in this format are not wrong logic, they are a fan-out from a key that turned out not to be unique, or a filter applied before an aggregate when it belonged after. Say what you expect before you run it.
What to demonstrate
- Whether your row counts survive each join, and whether you notice on your own when they do not
- Deliberate handling of rows that fail to match, including whether the question needs an inner join or a left join with the non-matches kept and counted
- Whether NULLs are treated on purpose, given that a NULL compares equal to nothing and that COUNT of a column skips it
- Reaching a defensible answer inside the window instead of a refined one after it
How to prepare
- Take a two-table schema, write a join that fans out on purpose, then fix it by collapsing the many-side to one row per key before joining. Repeat until the fix is reflex rather than recall.
- Write a funnel as one query and print the distinct user count at each stage, then confirm each stage is a subset of the one above it rather than assuming it
- Do a few timed runs in a plain text box with no autocomplete and no formatter, since assessment editors often have neither
Project Walkthroughs
reportedRounds outside the standard loop often open with something deliberately under-specified: a loose business problem, an open question about a product area, a dataset described in one sentence. The common failure is surveying, listing six plausible approaches and committing to none of them. The thing that separates a strong answer is scoping out loud. State what you are treating as the goal, name the metric you would move, say what you are choosing not to do and why, then take one path through to an actual answer. An interviewer can follow you down a narrow path. Nobody can grade a menu.
What to demonstrate
- Whether you turn an ambiguous prompt into a stated question with a measurable outcome before doing any work
- The judgement visible in what you cut, and whether you say why you cut it rather than silently dropping it
- Whether you land on a concrete recommendation with its caveat attached, rather than an unranked set of options
How to prepare
- Take three vague prompts, such as 'is this feature working', 'why did retention drop', and 'should we expand into a new segment'. For each, write one sentence of goal, one primary metric with its window, and two things you are explicitly not doing.
- Practise giving the recommendation first and the reasoning second, in five minutes. Loosely defined rounds are usually time-boxed, and an answer that arrives last often does not arrive.
- Keep a running assumption list as you talk, on paper or in the shared doc, so the interviewer can challenge one assumption instead of your whole answer.
Behavioral Interviews
reportedThis round decides whether you owned a decision or watched one happen nearby. Interviewers for data roles listen for the point where the analysis stopped being a report and started changing what someone did, so build each story around that hinge: what was going to happen by default, what you found, and what happened instead. The most common weakness is a story that ends at delivery. If you can name the decision your work changed and the number that moved because of it, most follow-ups become easy.
What to demonstrate
- Whether the decision was yours to influence, or whether you are narrating a team outcome in the first person
- The counterfactual: what would have been done without your analysis, and why that default was worse
- How far your involvement ran past the handoff, and whether you checked that the change did what you predicted
How to prepare
- Pick three projects and write one sentence for each naming the decision-maker, the choice in front of them, and what they chose after seeing your work. If you cannot name a person and a choice, the story is not ready for this round.
- Reconstruct the baseline for your strongest project from the original query or dashboard rather than memory, so the before-number survives a follow-up asking where it came from.
- Prepare an honest version of a project where your recommendation was overruled, including what you did with the analysis afterwards.
Panel Interview
reportedA day of back-to-back interviews samples your floor, not your ceiling. Four hours in, the habits that carry a good answer are the first to go: restating the question before solving it, asking what the data would have to look like, checking a number before quoting it. What the day decides is whether the tired version of you is still someone to leave alone with an ambiguous problem. The round that sinks a candidate is usually not the hardest one. It is the one immediately after the round that went badly.
What to demonstrate
- Whether the late rounds get the same clarifying questions as the first one, or whether you start answering immediately to save effort
- Whether a weak answer stays in the room it happened in, instead of following you into the next conversation as apology or distraction
- Whether the quality of your questions holds up, since fatigue removes curiosity about the problem before it removes knowledge of the method
How to prepare
- Rehearse the length, not just the content: book four mock interviews of different types in one afternoon with short gaps, because the one you need to observe is the fourth
- Put the two or three questions you ask at the start of any problem on a card in front of you, so that under fatigue it is a habit you run rather than a decision you make
- Decide in advance what the gap between rooms is for: water, one line of notes on anything you promised to follow up, and an explicit close on the round that just ended so it does not travel
- Prepare a different closing question for each interviewer, so the end of a long day does not produce the same one four times
PracHub editorial advice for the preparation topics above.
Computing average inventory from period-end snapshots
Shipments cluster before period close, so the month-end on-hand position is systematically the lowest point of the month; turns computed against it are biased high and days of supply biased low, frequently by ten to twenty percent, and the bias grows precisely when close-period push is strongest. The same shape of error appears when a daily snapshot is joined to shipment events on date equality: a SKU with several legs on one day fans the snapshot out and multiplies the valued inventory. Average across every daily snapshot in the window for the denominator, and join snapshots to events with an explicit as-of condition and a row-count check at each grain before aggregating.
Attributing demand variability to the node where it is observed
Order variance amplifies as it moves upstream: batching to a truckload, minimum order quantities, forecast-driven ordering and promotional pull all convert smooth end demand into lumpy replenishment orders, so a plant can see a coefficient of variation several times that of the underlying consumption. Diagnosing the plant's schedule instability as a plant problem then produces interventions that cannot work, because the generating process sits one or two echelons downstream. Measure the bullwhip ratio explicitly (variance of orders placed by a node over variance of demand it received) at each echelon, and fix the ordering rule at the node where the ratio jumps rather than the node where the pain is felt.
Ending an analysis without a recommendation or next step
Close with what you would do and what would change your mind, stated as a condition you can check later. If the evidence is genuinely inconclusive, recommend the specific next measurement and say what it costs in time or exposure.
Generalising beyond the population the sample actually supports
State the frame the sample was drawn from and where it diverges from the population you want to talk about: time window, platform, geography, opt-in. If a group is excluded from the frame, either weight to known margins or narrow the claim rather than quietly extending it.
Choose a category, try a prompt, then open its approach, worked solution or follow-up when you need it.
How do you prevent overfitting in deep neural networks?
How do you prevent overfitting in deep neural networks?
Approach
- Say how the offline result would be validated online before it is trusted.
- Check what information would not exist at prediction time, and exclude it.
- Pick an evaluation metric that matches the cost of each error type, not a default.
Follow-up
- How would you choose the decision threshold, and who owns that choice?
- Where could label leakage enter this setup?
Walk me through a machine learning project you completed from end to e…
Walk me through a machine learning project you completed from end to end. What challenges did you face, and how did you resolve them?
Approach
- 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.
- Pick an evaluation metric that matches the cost of each error type, not a default.
Follow-up
- Where could label leakage enter this setup?
- How would you choose the decision threshold, and who owns that choice?
As-of join snapshots to order lines without merge_asof
You have fct_order_line (order_line_id, sku_id, ship_from_location_id, actual_ship_at_utc, shipped_qty, shipment_id) and fct_inventory_daily (inventory_date, sku_id, location_id, available_qty). For every shipped line, attach available_qty from the latest snapshot strictly before the ship date for that exact sku and node, plus the shipment's delivery timestamp from fct_shipment_leg (leg_id, shipment_id, delivered_at_utc, leg_status). The output must have exactly one row per input order line. pandas.merge_asof is not available; implement the as-of match yourself.
Approach
- Collapse fct_shipment_leg to one row per shipment_id before joining anything. A shipment is one or more legs, so a direct line-to-leg join multiplies every line by its leg count and quietly inflates every sum computed afterwards.
- Define the shipment-level delivery as the maximum delivered_at_utc across its legs, and set it null if any leg is not delivered, since the customer has the order only when the last leg lands.
- Sort the snapshot frame by (sku_id, location_id, inventory_date) and the line frame by the same key plus ship date, then use np.searchsorted with side='left' on the sorted dates within each key group to find the insertion point; the matching snapshot is the row at position minus one.
- Guard the boundary explicitly: an insertion point of 0 means no snapshot exists before that ship date, and the result must be null rather than the first available future snapshot. Count those lines and report them.
- Assert the output row count equals the input line count immediately after each merge, before any aggregation. Row-count drift is the only symptom a silent fan-out produces, and it is invisible once the frame has been grouped.
Worked solution 30 min
- Aggregate legs by shipment_id to produce delivered_at_utc as the max, leg_count, and a delivered_all flag from leg_status.
- Left join that one-row-per-shipment frame onto fct_order_line by shipment_id and assert the row count is unchanged.
- Sort snapshots and lines by the composite key, then compute per-group insertion points with np.searchsorted and subtract one.
- Mask positions below zero to null, take available_qty at the remaining positions, and attach it as available_qty_prior.
- Validate on a single sku-location pair by printing its snapshot dates and one line's ship date and confirming the chosen date by eye.
Follow-up
- The snapshot is taken at the node's local end of day and actual_ship_at_utc is UTC. Where does that break the strictly-before rule and what do you need from dim_location to fix it?
- How would you extend this to allocate each leg's linehaul cost back to the order lines it carried?
- What changes if a line ships in two partial shipments?
Write a query to find the second-highest energy-consuming household fr…
Write a query to find the second-highest energy-consuming household from a customer transactions table.
Approach
- Handle the rows that do not match: a LEFT JOIN with a NULL check is usually the question.
- 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.
Follow-up
- How does the query change if the join becomes one-to-many?
- What breaks if events arrive late or out of order?
What is the difference between a `LEFT JOIN` and an `INNER JOIN` in SQ…
What is the difference between a LEFT JOIN and an INNER JOIN in SQL?
Approach
- Handle the rows that do not match: a LEFT JOIN with a NULL check is usually the question.
- 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.
Follow-up
- What breaks if events arrive late or out of order?
- How would you verify this result without re-running the same query?
How do you optimize a slow-performing SQL query?
How do you optimize a slow-performing SQL query?
Approach
- Check whether any join is one-to-many before aggregating, or the sums inflate.
- Compute rates by summing numerator and denominator separately, never by averaging rates.
- 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?
Explain the concept of database normalization and why it is important.
Explain the concept of database normalization and why it is important.
Approach
- 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.
- Compute rates by summing numerator and denominator separately, never by averaging rates.
Follow-up
- What breaks if events arrive late or out of order?
- How would you verify this result without re-running the same query?
Lag-7 forecast accuracy and bias at weekly grain
fct_inventory_daily has one row per sku_id x location_id x inventory_date carrying demand_qty, forecast_qty_lag7 (the forecast for that date as published seven days earlier, NULL before the series existed) and stockout_flag. Compute trailing eight-week WMAPE and signed weighted bias at the sku x location x week grain the ordering decision uses: aggregate demand and forecast to the week first, then form errors on the weekly totals. Exclude any week containing a NULL forecast day and report how many cell-weeks that removed. Return one accuracy row per ABC class.
Approach
- Roll daily rows to sku-location-week with SUM(demand_qty) and SUM(forecast_qty_lag7), carrying COUNT(*) FILTER (WHERE forecast_qty_lag7 IS NULL) so a partial week is visible rather than quietly summing to an artificially low forecast.
- Drop cell-weeks with any NULL forecast day and emit the dropped count as a column: an accuracy figure without its exclusion count cannot be checked by anyone.
- WMAPE = SUM(ABS(weekly_demand - weekly_forecast)) / NULLIF(SUM(weekly_demand), 0). Bias = SUM(weekly_forecast - weekly_demand) / NULLIF(SUM(weekly_demand), 0), signed, with cancellation as the whole point of reporting it next to WMAPE.
- Join dim_sku for abc_class and group there by re-summing both sides. Averaging per-SKU WMAPE gives a number weighted by nothing in particular.
- Report the share of retained cell-weeks containing a stockout day beside the result, because demand_qty on those days is censored at what could be supplied and the error is being scored against a truncated actual.
Worked solution 25 min
- CTE one: GROUP BY sku_id, location_id, DATE_TRUNC('week', inventory_date) with SUM(demand_qty), SUM(forecast_qty_lag7) and the NULL-day count.
- CTE two: split the cell-weeks into retained (null_days = 0) and excluded, and keep COUNT(*) of each.
- Join dim_sku on sku_id for abc_class, then aggregate the retained set per class with the two ratio formulas.
- Compute the stockout exposure share over the same retained set and attach it to each class row.
- Print retained count, excluded count and window bounds alongside every ratio.
Follow-up
- Why is MAPE unusable on a C-class item at a forward-stocking location, and what do you score it with instead?
- WMAPE improves sharply when you move from sku-location-week to sku-region-week. Which of the two belongs in the planning review, and why?
- Bias is +6 percent and WMAPE is 45 percent. What does that pair tell you to investigate first?
An OEE target that makes the plant run the wrong SKUs
A constraint work centre is put on an OEE target. Using fct_production_run (planned_production_seconds, run_seconds, unplanned_downtime_seconds, changeover_seconds, ideal_cycle_seconds_per_unit, planned_qty, total_qty, good_qty, scrap_qty, rework_qty, downtime_reason_code, run_status) and fct_inventory_daily (on_hand_qty, demand_qty), define OEE precisely, decide whether planned changeover sits inside or outside the denominator, and name the guardrail that stops the target being met by running long batches of whatever is easiest. Defend the conflict rather than hiding it.
Approach
- Write the metric exactly: SUM(good_qty * ideal_cycle_seconds_per_unit) over SUM(planned_production_seconds), re-summed for every roll-up, and show that it factors into availability times performance times quality so a movement is attributable before anyone argues about it.
- Settle the changeover question in writing, because it decides gameability. Leaving planned changeover inside the denominator makes OEE fall whenever variety rises, which is the behaviour the target would then buy; excluding it protects mix but hides real capacity loss. Exclude it and carry changeover_seconds as its own reported series.
- Use ideal_cycle_seconds_per_unit per SKU and work centre rather than a line average, since a line-average nameplate rate turns performance into a function of which SKU happened to run.
- Set the guardrail on demand-weighted schedule adherence, good_qty against planned_qty at SKU level weighted by what the network is short of, paired with finished-goods days of supply for the SKUs that were skipped.
- State the conflict plainly: a plant maximising OEE lengthens runs and drops small-volume SKUs, which raises the score and lowers service, so the two metrics are published together and the target is set on the pair, not on OEE alone.
- Scope the metric to constraint work centres, because OEE on a non-constraint rewards making inventory nobody is waiting for, and scope is part of the definition rather than a footnote.
Worked solution 25 min
- Compute the factors per work centre per shift: availability = run_seconds / planned_production_seconds, performance = (total_qty * ideal_cycle_seconds_per_unit) / run_seconds, quality = good_qty / total_qty.
- Verify the identity on every row: the product equals good_qty * ideal_cycle_seconds_per_unit / planned_production_seconds.
- Build the mix control as SKU share of run_seconds per period, then split a period-over-period OEE move into within-SKU rate change and mix change.
- Build the guardrail from demand-weighted schedule adherence plus days of supply from fct_inventory_daily for the SKUs on the plan.
- Write the target as a pair with an explicit adherence floor, and state the decision rule for the case where OEE rises while adherence falls.
Follow-up
- Availability rose and performance fell in the same week. Give two explanations and the query that separates them.
- Where does rework_qty enter your definition, and what does that choice reward?
Success metric for opening one forward-stocking node
The network will open one forward-stocking location at echelon 3, replenished by an existing regional DC. There is one treated node and about thirty candidate comparison nodes. Using fct_order_line (ordered_at_utc, actual_delivery_at_utc, ship_from_location_id, line_status), fct_inventory_daily (on_hand_qty, standard_cost_cents) and fct_shipment_leg (direction, shipped_units, linehaul_cost_cents, accessorial_cost_cents, delivered_at_utc), define the primary metric, the guardrail that catches the working-capital cost of de-pooling, and the readout design. Say what this sample cannot detect.
Approach
- Anchor the primary on what the node is for: P90 order cycle time for the lines it serves, taken over the pooled line-level distribution rather than as a mean of daily percentiles, with never-delivered lines reported as a censored count instead of dropped.
- Make the inventory guardrail network-wide, not node-local. Splitting one pooled stock into two locations raises total safety stock at the same service level; under independent demand, equal service and equal lead times the square-root law puts the factor at sqrt(2) on the safety component, so measure parent and new node combined at cost.
- Add a freight guardrail across both the new inbound interfacility leg and the outbound legs, because the opening adds a movement that any node-local outbound cost metric will miss entirely.
- Choose a design suited to one treated unit: a synthetic control or matched-pair comparison built from donor nodes on pre-period cycle time, volume, SKU mix and lane structure, with the donor pool and matching variables frozen before the readout.
- Set the burn-in at two full replenishment cycles, and book the fill-and-build inventory into the cost side rather than letting it contaminate the service read.
- State the limit rather than implying precision: with one treated unit there is no cross-unit variance, so a placebo distribution over the donors is the only inferential statement available, and effects smaller than the donor spread cannot be distinguished from node noise.
Follow-up
- A donor node runs a promotion during the post period. What do you do, and at what point did you have to decide it?
- The service gain is real and the cost guardrail is also up. Who decides, and on which number?
On-time rate improved the week the promise field changed
Customer on-time delivery jumped from 88 percent to 95 percent in one week and has stayed there. That week a release began writing re-confirmed dates into fct_order_line.promised_delivery_date, while original_promised_delivery_date still carries the first promise and both columns are populated for all history. Using fct_order_line (actual_delivery_at_utc, promised_delivery_date, original_promised_delivery_date, line_status) and dim_location.timezone, determine how much of the seven points is real. Deliverable: two numbers that sum to seven and one sentence of recommendation.
Approach
- Recompute the whole series twice on identical rows, once against promised_delivery_date and once against original_promised_delivery_date. Both columns exist for all history, so the counterfactual is free and the split is arithmetic rather than an estimate.
- Read the gap between the two series in the weeks before the release. If it was near zero and opens abruptly in the release week, the jump is the measurement changing. If the original-promise series also rises, that component is real and should be credited.
- Measure the mechanism directly rather than inferring it: by week, the share of lines where promised_delivery_date > original_promised_delivery_date and the mean slip in days. Any metric that can be satisfied by moving its own target is not measuring service.
- Handle the date-versus-timestamp comparison explicitly. actual_delivery_at_utc is a UTC instant while the promise is a local date, so convert using dim_location.timezone and compare against local end of day. Doing the comparison in UTC moves an entire timezone's deliveries across the boundary and invents a second, smaller step.
- Recommend the original-promise definition for the published metric and keep the re-promise series as a separate operational measure, stating that the two answer different questions rather than that one is simply wrong.
Follow-up
- Re-promises are legitimate when a customer agrees to the new date. How would you let those count without reopening the loophole?
- What automated check would catch the next definition change before it reaches a chart?
- Supplier inbound on-time has the same structure. Where is its equivalent loophole, and what does the metric tree already say about it?
Roughly 90 minutes a night on weekdays with one longer weekend block. The plan deliberately cuts scope rather than compressing everything, on the assumption that finishing one thing a night beats half-starting four.
Prepare, practise & reflect
One practical outcome each day. Spend longer where you need it.
0 / 7 done01Fix the scope and set a baseline
- Read the role description and write the three things the loop will almost certainly test, then write an explicit not-doing list for everything else and keep it visible all week.
- Take one 20-minute SQL prompt and one 10-minute metric question cold, and write the single sentence that says what blocked each attempt, since that sentence is what decides which two topics get the most evenings.
- Set the week's one rule: one problem finished to completion every night, including the night you only have 40 minutes.
Deliverable: A one-page scope with an explicit not-doing list and two cold attempts, each carrying one sentence on what blocked it.
Practice prompt ↗Practice prompt ↗Worked solution ↗02One query pattern, written three times
- Choose the single pattern most likely to appear (a cohort retention grid, or a funnel counted by user) and write it three times from a blank file rather than editing the previous attempt.
- On the third attempt, write the grain of every CTE as a comment before writing its body.
- Stop at 90 minutes even if the third version is imperfect, and write the one thing you would fix with another hour.
Deliverable: Three independent versions of the same query plus a note on what changed between them.
Practice prompt ↗Practice prompt ↗03Only the statistics you will be asked to defend
- Write, in under 200 words, how you would decide whether a difference between two groups is real: the test, its assumptions, and what you would switch to when an assumption fails.
- Compute a 95 percent confidence interval for a difference in proportions by hand on realistic numbers, then write in one sentence what changes if the two samples are paired rather than independent.
- Write your answer to "what does a p-value mean", check it against a definition, and delete the version that describes it as the probability the hypothesis is true.
Deliverable: A 200-word written answer and one hand-computed interval you can reproduce under pressure.
Practice prompt ↗Practice prompt ↗04One case, and the assumptions holding it up
- Answer one product case aloud in 20 minutes with a recording running, then listen back with a pen and mark every claim you asserted without saying what it rested on: an assumed user behaviour, an assumed data source, an assumed baseline rate, an assumed grain.
- Pick the three assumptions the recommendation actually depends on, write how you would check each one against data, and say which one being wrong would flip the recommendation rather than merely weaken it.
- Write the four-step structure you used onto a card small enough to hold in working memory when you are nervous.
Deliverable: One recording, three load-bearing assumptions each with a written check, and a four-step structure card.
Practice prompt ↗Practice prompt ↗Worked solution ↗05Your own work, timed
- Write a 90-second version and a four-minute version of your main project, and time both out loud rather than reading them.
- Prepare answers to the two follow-ups that always come: what you would do differently, and how you knew it worked.
- Put one number in the first sentence and be able to say exactly where that number came from and what it excludes.
Deliverable: Two timed narratives with one defensible number in the opening line.
Practice prompt ↗Practice prompt ↗06The one full rehearsal, in a longer weekend block
- Run a 60-minute mock covering query work, a case and a behavioural question in a single sitting with no breaks, because sustained attention is the thing evenings have not trained.
- Immediately afterwards, and before hearing any feedback, write the three moments you lost the thread.
- Spend the rest of the block only on those three moments, and on nothing you merely feel shaky about.
Deliverable: Mock notes naming three failure moments with a specific fix written under each.
Practice prompt ↗Practice prompt ↗07Taper
- Write the 20-minute warm-up you will actually do on the morning of the interview: one query you can already write from a blank file, one metric you can define out loud, and nothing you have never seen before.
- Re-read only your own notes from this week, and open no new material.
- Write down the logistics: the tool you will be asked to work in, whether lookups are allowed, and the sentence you will use when you do not know something.
Deliverable: A one-page card holding the case structure, the project numbers, and the logistics.
Practice prompt ↗Practice prompt ↗Worked solution ↗Expand any day for tasks and deliverables. Your progress is saved on this device.
Saying no well is a senior skill and it is rarely rehearsed. Think of a time you told someone their analysis was not worth doing, or that the experiment could not answer their question at the sample size available. Explain what you offered instead. Refusal without an alternative reads as obstruction rather than judgement.
How do you handle missing or null values in a database before feeding …
How do you handle missing or null values in a database before feeding data into a machine learning pipeline?
Approach
- Name the disagreement or constraint, and how you resolved it with evidence.
- Quantify the outcome, including what you would not claim credit for.
- State the situation in two sentences and spend the rest on your reasoning.
Follow-up
- How did you know the outcome was caused by your change?
- What would you do differently if you ran that project again?
How do you handle highly imbalanced datasets when building classificat…
How do you handle highly imbalanced datasets when building classification models?
Approach
- Close with what you would do differently, concretely.
- State the situation in two sentences and spend the rest on your reasoning.
- Quantify the outcome, including what you would not claim credit for.
Follow-up
- What would you do differently if you ran that project again?
- What did you decide not to do, and why?
Scope a vague request about rising inventory
A planning director opens with: inventory is up twelve percent on flat shipments, get me an analysis by Friday. You have fct_inventory_daily (inventory_date, sku_id, location_id, on_hand_qty, standard_cost_cents) and fct_order_line (shipped_qty, unit_cogs_cents, requested_ship_date). Nothing else is specified: not the window, not the comparison basis, not whether the twelve percent is units or value. State the three questions you would ask before writing any SQL, name the decision each answer changes, and describe the first cut you would run if nobody answers you before Friday.
Approach
- Ask what decision hangs on the answer: a buy stop, a write-off provision, a policy review and a board slide all need different cuts, and the director usually has one of them in mind.
- Pin the measurement before the cause: units or value, which two windows are being compared, and whether the denominator is average daily on-hand or a period-end snapshot, since a month-end denominator biases turns high by ten to twenty percent and can manufacture the whole movement.
- Ask which part of the network is in scope, because echelon matters: a build at echelon 2 ahead of a promotion and a build of blocked_qty at a plant are different problems with different owners.
- Commit to a default if no answer arrives: value at standard cost, averaged across every daily snapshot in both windows, cut by echelon, then by abc_class and xyz_class, then by lifecycle_status to separate phase_out stock from active cover.
- Say out loud what the first cut cannot settle, so the director is not surprised: it localises the build but does not attribute it to forecast bias, a lot-size change or a supplier pulling orders in.
Follow-up
- The build is concentrated in one product family at two DCs. What are your next two queries, and what would make you stop calling it a planning problem?
- The director wants the number by Friday and the cause by Friday. Which do you drop, and how do you say so?
- 01
How do you handle missing or null values in a database before feeding data into a machine learning pipeline?
- 02
How do you handle highly imbalanced datasets when building classification models?
- 03
A planning director opens with: inventory is up twelve percent on flat shipments, get me an analysis by Friday. You have fct_inventory_daily (inventory_date, sku_id, location_id, on_hand_qty, standard_cost_cents) and fct_order_line (shipped_qty, unit_cogs_cents, requested_ship_date). Nothing else is specified: not the window, not the comparison basis, not whether the twelve percent is units or value. State the three questions you would ask before writing any SQL, name the decision each answer changes, and describe the first cut you would run if nobody answers you before Friday.
Is this an official Nextera Energy Services interview guide?
No. It is PracHub's own research and practice material for the Data Scientist role at Nextera Energy Services. Rounds and questions reflect what candidates have reported, not a process Nextera Energy Services has published, and they change over time. Confirm the current format and scope with your recruiter.
PracHub interview research ↗How technical is the interview process for a Data Scientist at Nextera Energy Services?
The process is highly rigorous. While early stages assess basic SQL and behavioral fit, final rounds—especially the onsite panel—can involve deep interrogations on machine learning theory, statistical modeling, and database design.
PracHub interview research ↗What is the format of the initial on-demand video interview?
This screen typically consists of about 6 standard behavioral and project-related questions. You are generally given 1 minute to think about each question and 3 minutes to record your response.
PracHub interview research ↗Is domain experience in the energy sector required?
While prior experience in utilities or green energy is a strong differentiator, it is not strictly required. Demonstrating strong foundational data science skills and a quick ability to learn complex physical systems is highly valued.
PracHub interview research ↗How should I prepare for the final round project presentation?
Select a project where you had significant ownership and technical depth. Prepare a clear, professional slideshow that walks through the problem, your methodology, the technical trade-offs you made, and the ultimate business outcome.
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
Explore related engineering environments and compare the preparation focus.
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