Interview guides · Coupa

Coupa Data Scientist Interview Questions & Guide 2026

Coupa data scientist prep: probability, sampling distributions, and stating the assumption behind every test you run.

The 60-second brief

As a Data Scientist at Coupa, you sit at the intersection of complex financial data and actionable intelligence. Your work is fundamental to the Coupa Business Spend Management (BSM) platform, where you transform massive datasets into insights that help organizations optimize their spending, manage supply chain risks, and improve operational efficiency. You are not just building models; you are solving real-world economic problems for global enterprises.

Coding rounds for this role are usually data-manipulation shaped rather than data-structure shaped: group-bys, joins, time windows, ranking within a partition. Confirm the format before spending a week on graph traversal.

Focus areas

Separate contracted seats from actively used seats
Power experiments for heavy-tailed account revenue
Read NRR on a fixed account cohort

Overview

The role demands a balance of rigorous technical application and business acumen. You will engage with high-stakes projects involving NLP, predictive modeling, and optimization algorithms. Because Coupa operates at a significant scale, your contributions directly impact how businesses make multi-million dollar decisions. You will collaborate closely with product managers and engineers to ensure that your models are not only statistically sound but also scalable and production-ready.

Candidates report that the interview process can vary significantly by team. Be prepared to pivot between high-level architectural discussions and granular technical implementation based on your interviewer's background.

The Coupa interview process

Coupa candidates report 5 rounds · ≈ 4-6 weeks. The stages below are what candidates describe, not a published process.

Recruiter Engagement

A 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 Screens

Much of what gets scored here happens out loud while you type. Nobody can see your reasoning inside a half-written query, so five silent minutes read as being stuck even when they are not. State the plan in plain language first: which tables, what grain you are aggregating to, and the one filter that defines the population. Then write it. The narration doubles as insurance, because a wrong plan gets caught early and cheaply while a wrong query gets caught at the end with no time left to redo it. A timed statistics section, where one exists, is a separate test with its own clock.

What to demonstrate

Whether the query you write matches the plan you just described
What you do with a hint, meaning whether the correction gets absorbed or the first approach gets defended
Whether you can debug your own wrong output by reading the result set and naming which part of the query produced the anomaly

How to prepare

Solve three problems while screen-sharing into a recording, then watch it back and mark every stretch longer than thirty seconds where you said nothing
Practise compressing the plan into one sentence before typing, then check afterwards whether the finished query actually matched it
Time yourself on statistics questions that carry a business reading, such as what a confidence interval does and does not claim, rather than re-reading notes without a clock

Deep-Dive Project Review

An added round often puts you in front of someone outside the core hiring team: a partner engineer, a product owner, a domain expert, sometimes a more senior manager. The question they are really asking is not whether you can do the work but whether they would trust a number that came from you. That changes what a good answer looks like. Lead with what the decision cost and what it changed, keep the method available but not central, and be plain about the limits of your evidence. Overstating a result is the fastest way to lose this round.

What to demonstrate

Whether you can explain a technical choice to someone who will never read your code, without either flattening it into nothing or hiding inside jargon
Honesty about evidence strength: what the analysis establishes, what it only suggests, and what it cannot say at all
How you take disagreement, specifically whether you update on a good objection, hold your position with reasons, or fold on contact

How to prepare

Write the two-sentence version of your most technical project for a non-specialist, then check that neither sentence needs a method name to make sense.
For one result you are proud of, write the strongest objection someone could raise and a response that concedes the part of it that is correct.
Prepare one decision that turned out to be wrong: how you found out, what it cost, and what you changed afterwards. A senior cross-functional interviewer asks for this more often than a technical one does.

Behavioral Assessment

This 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.

Final Interview Rounds

A 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

Where candidates lose points

Treating raw request or usage volume as engagement

Most traffic in this domain is emitted by machines. Continuous-integration pipelines, scheduled batch jobs, synthetic monitors, backfills and client retries can all grow by an order of magnitude from one configuration change made by one engineer, and none of it represents a new decision to use the product. The inversion is what makes it dangerous: when the platform degrades, clients retry, so error-driven retry volume rises at the exact moment the customer is most likely to leave, and an engagement dashboard built on raw counts shows growth immediately before a churn. Filter on traffic_class and on successful status before anything else, and keep failed-request volume as its own separate series.

Reading consumption metrics before the metering lag window has closed

Usage pipelines land late and correct themselves, which is exactly what is_restated and restated_at record. A dashboard queried on day T sees a partially populated tail for the last several days, so the most recent points always slope downward and always look like a regression. Analysts then explain the artefact, and sometimes ship a change to fix it. Establish the empirical settling time by measuring how much a given usage_date's total moves between first_written_at and its final value, exclude that many trailing days from every reportable figure, and never compare a fresh period against a settled one.

Reporting a mean for a heavy-tailed metric without saying what it hides

For spend, session length or items per order, a small fraction of units carries most of the total, so the mean has a wide standard error and one account can move it. Fix the handling before you see the result: cap or winsorise at a pre-declared percentile, and report the median or the share above a threshold next to the mean. Capping changes the estimand, so say which question the capped number answers, and check how much of any difference comes from the top 0.1 percent of units.

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.

Questions & practice

Why do we use optimizers in neural networks rather than standard gradient descent?

Medium · machine learning and modelling

Approach

Set a baseline first, so any model has something honest to beat.
Pick an evaluation metric that matches the cost of each error type, not a default.
Say how the offline result would be validated online before it is trusted.

Follow-up

Where could label leakage enter this setup?
How would you choose the decision threshold, and who owns that choice?

Describe the process of feature engineering for a BSM-related predictive task.

Medium · machine learning and modelling

Approach

Set a baseline first, so any model has something honest to beat.
Pick an evaluation metric that matches the cost of each error type, not a default.
Check what information would not exist at prediction time, and exclude it.

Follow-up

Where could label leakage enter this setup?
What would you monitor after launch to know the model is still valid?

Sessionise an API event stream with a 30-minute inactivity gap

Medium · sessionisation · event-streams · pandas

fct_api_request arrives as a DataFrame with account_id, user_id, request_at (tz-aware UTC), traffic_class and http_status, roughly 5 million rows. Assign a session_id to every human-attributable request: drop rows where user_id is null or traffic_class is in ('ci','synthetic_monitor','load_test'), then open a new session whenever the gap since that user's previous remaining request exceeds 30 minutes. Return the filtered frame plus session_id, and a per-session summary with user_id, account_id, session start, session end and request count. Do not loop over rows.

Approach

Settle the filter-then-gap ordering before writing code. Removing CI and synthetic rows changes the gaps, so sessionising the raw stream and filtering afterwards is a different answer; the definition given filters first, and the two diverge most for accounts whose CI runs every ten minutes.
Sort once by (user_id, request_at) with a stable kind, then gap = df.groupby('user_id', sort=False).request_at.diff(). The first row of each user yields NaT, which is exactly the boundary condition you want rather than a special case to patch.
new_session = gap.isna() | (gap > Timedelta(minutes=30)); session_id = new_session.cumsum(). The cumsum runs over the whole sorted frame and therefore produces globally unique ids in one pass; a per-user cumcount collides across users and forces a composite key on every downstream join.
Build the summary with a single groupby('session_id').agg(...). user_id and account_id can be carried with 'first' only because the sort key groups them — state that dependency, since it silently breaks if someone later re-sorts the frame.
Decide explicitly what a session means when one user_id holds memberships in several accounts: either add account_id to the sort and group keys, or document that sessions may cross accounts. Leaving it undecided produces sessions whose account_id is whichever row sorted first.

Worked solution (30 min)

human = df[df.user_id.notna() & ~df.traffic_class.isin(['ci','synthetic_monitor','load_test'])].sort_values(['user_id','request_at'], kind='mergesort').reset_index(drop=True)
gap = human.groupby('user_id', sort=False).request_at.diff(); human['session_id'] = (gap.isna() | (gap > pd.Timedelta(minutes=30))).cumsum()
summary = human.groupby('session_id').agg(user_id=('user_id','first'), account_id=('account_id','first'), start=('request_at','min'), end=('request_at','max'), n_requests=('request_at','size')).reset_index()
assert summary.n_requests.sum() == len(human) and human.groupby('session_id').user_id.nunique().max() == 1

Expected result: human gains a dense session_id increasing in (user_id, request_at); summary has exactly one row per session, n_requests sums to len(human), and every within-session consecutive gap is at most 30 minutes while every same-user between-session gap exceeds it.

Verify it yourself

summary.n_requests.sum() == len(human); no request is lost or double-counted.
human.groupby('session_id').user_id.nunique().max() == 1.
human.groupby('session_id').request_at.diff().max() <= pd.Timedelta(minutes=30).
Shifting every timestamp by a constant offset leaves the session count unchanged.

Follow-up

Where does 30 minutes come from, and how would you pick it from this data instead of from convention?
An engineer reused their personal key for a nightly batch job, so machine traffic carries a human user_id. How would you detect that, and should those requests form sessions?
How much does the session count change if you sessionise before dropping CI traffic rather than after?

These questions test your proficiency in Python and your understanding of data structures, which is a common requirement for productionizing models.

Medium · sql and data manipulation

Approach

Say which table is the grain you start from, and join outward from it.
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.

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 SQL query to solve a specific data aggregation problem without knowing the schema beforehand.

Medium · sql and data manipulation

Approach

State the window function and its partition and ordering out loud before writing it.
Handle the rows that do not match: a LEFT JOIN with a NULL check is usually the question.
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?

How would you implement a multi-threaded application in Python for data processing?

Medium · sql and data manipulation

Approach

Say which table is the grain you start from, and join outward from it.
Handle the rows that do not match: a LEFT JOIN with a NULL check is usually the question.
Compute rates by summing numerator and denominator separately, never by averaging rates.

Follow-up

How would you verify this result without re-running the same query?
How does the query change if the join becomes one-to-many?

Running commitment burn-down and the date consumption crosses it

Medium · window functions · running totals · committed consumption

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.

Worked solution (30 min)

Build terms: SELECT account_id, committed_amount_cents, term_start_date, term_end_date FROM fct_subscription_period WHERE is_current AND pricing_model = 'committed_consumption' AND committed_amount_cents IS NOT NULL.
Build daily: join fct_usage_daily to terms on account_id with usage_date inside the term, then GROUP BY account_id, usage_date summing net_amount_cents.
Add running_cents with SUM(...) OVER (PARTITION BY account_id ORDER BY usage_date ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW).
Aggregate per account with MIN(usage_date) FILTER (WHERE running_cents >= committed_amount_cents) AS crossing_date, then compute the elapsed fraction with the NULLIF guard.
LEFT JOIN that summary back onto terms so every committed account appears, and verify the last running_cents per account equals a plain SUM over the term.

Expected result: A per-account-per-day series of running_cents inside the current term, plus one summary row per committed account carrying crossing_date and term_elapsed_fraction, with crossing_date NULL exactly for accounts whose final running total is below committed_amount_cents.

Verify it yourself

The maximum running_cents per account equals SUM(net_amount_cents) over that account's term computed independently.
crossing_date is NULL if and only if the final running total is below the commitment.
term_elapsed_fraction falls in [0, 1] for every non-null crossing date.
running_cents is non-decreasing within each account, which fails immediately if a negative credit row was included without comment.

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?

How do you optimize a script for large-scale data manipulation?

Medium · product sense and metrics

Approach

Decompose the metric into the rates that drive it, and say which one you would check first.
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

What would you do if the primary metric and the guardrail moved in opposite directions?
How would you detect that the metric is being gamed rather than genuinely improving?

How do you determine when a model is ready for production, and what metrics do you prioritize?

Medium · product sense and metrics

Approach

Name one primary metric, then the guardrail that stops it being gamed.
State what result would change your recommendation, so the answer is falsifiable.
Decompose the metric into the rates that drive it, and say which one you would check first.

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?

Explain the differences between RNNs and Transformers in the context of sequence modeling.

Medium

Approach

State your assumptions explicitly before working the problem.
Clarify what is being asked and what a complete answer would contain.
Work from the decision backwards to the evidence you would need.

Follow-up

What assumption would you test first?
How would you know your answer was wrong?

Estimate a staggered overage-price change without a control arm

Hard · difference-in-differences · synthetic-control · staggered-adoption

On 2026-03-01 the overage rate for sku_code = 'ingest_gb' rose 18% for accounts whose dim_account.billing_country falls in six countries. Each account moves to the new rate at its own renewal, so treatment switches on across twelve months. Randomisation was never available. Using fct_usage_daily and fct_subscription_period, estimate the effect on net_amount_cents per account and on gross logo retention. Specify the estimator, the identifying assumption and how you test it, how you handle accounts that churn out of the panel, and how you do inference with six treated countries.

Approach

Fix the estimand before the estimator. Effect on revenue per account and effect on retention are different questions, and the second determines whether the first is worth having: an 18% rate rise that raises revenue while costing renewals is a loss. Define both on a cohort frozen before 2026-03-01, carrying churned accounts at zero revenue rather than dropping them, so attrition cannot masquerade as a revenue effect.
Do not run a single two-way fixed-effects regression. With treatment switching on at different renewal dates and effects that vary with time since treatment, the two-way fixed-effects coefficient is a variance-weighted average that includes comparisons of later-treated accounts against already-treated ones, and those comparisons can carry negative weights. Use a cohort-by-cohort estimator such as Callaway-Sant'Anna, or a stacked event study using only not-yet-treated controls, and report the event-study path rather than one number.
Treat parallel trends as a maintained assumption to be probed, not a result to be shown. Plot pre-treatment lead coefficients with intervals and state that flat leads are consistent with the assumption rather than proof of it. Handle anticipation explicitly: accounts notified of the increase before their renewal can pull ingest volume forward, so exclude a pre-renewal anticipation window and check whether the leads move when you do.
When the six treated countries are dominated by a few large accounts, the group mean is not a stable object and difference-in-differences on it will be noisy in a way its standard error will not reflect. Build a synthetic control at the country level from a donor pool of untreated countries, weighted to match the pre-period trajectory of net revenue per account, and report the placebo distribution across donor countries instead of a conventional standard error.
Do inference at the level treatment was assigned, which is the country. Six treated clusters is far too few for cluster-robust standard errors, which are badly downward-biased below roughly forty clusters. Use a wild cluster bootstrap, and report a randomisation-inference p-value from placebo assignments of treated status alongside it.
Apply the domain's data hygiene or the whole estimate is contaminated: exclude is_internal accounts, exclude the trailing metering settling window so the final months are not artificially low, and join as-of to the fct_subscription_period version live on each date rather than to is_current, which would price last year's usage at this year's contract.

Worked solution (45 min)

Build a balanced monthly account panel from fct_usage_daily joined as-of to fct_subscription_period, excluding is_internal accounts and carrying churned accounts forward at zero revenue.
Define treatment cohorts by renewal month, estimate group-time average treatment effects, and aggregate to an event-study path with leads from -6 to -1 and lags from 0 to +11.
Fit a country-level synthetic control on pre-period net revenue per account and generate the placebo distribution by refitting on each donor country in turn.
Compute a wild cluster bootstrap p-value clustered on country and report the randomisation-inference p-value from the placebo distribution beside it.
Repeat the full path for gross logo retention, restricted each month to the renewal-eligible base plus the 45-day grace window.

Expected result: An event-study path whose lead coefficients are statistically indistinguishable from zero and whose lags are reported by months since renewal, with country-level inference from a wild cluster bootstrap and a placebo-based p-value from the synthetic control. A single pooled two-way fixed-effects number is not reported, and the retention path is computed only on the renewal-eligible base.

Verify it yourself

Treated and synthetic pre-period trajectories must track within the pre-period fit error. A poor pre-period fit cannot be rescued by a longer post-period and invalidates the comparison.
Gross logo retention must be computed on accounts whose term_end_date fell in the month, not on the whole panel. On annual contracts only about a twelfth of accounts can churn in any month, so the whole-base rate understates churn by roughly an order of magnitude.
At six treated clusters the wild cluster bootstrap p-value should be noticeably larger than the naive cluster-robust one. If they agree closely, the bootstrap was run on the wrong clustering variable.

Follow-up

Your event study shows a significant lead coefficient two months before renewal. Is that anticipation, a violation of parallel trends, or a coding error, and what distinguishes them?
Revenue per account rose 6% and gross logo retention fell 1.8 points. How do you combine those into a single recommendation, and over what horizon?
One treated country contains an account holding 20% of that country's revenue. What does that do to the synthetic control fit, and what would you do about it?

Blended gross margin fell while every segment improved

Medium · mix shift · simpsons paradox · as-of join

Blended gross margin across paying accounts fell from 71 percent to 66 percent over two quarters, yet margin improved inside every plan_tier and every deployment_model. You have fct_usage_daily (account_id, sku_code, usage_date, net_amount_cents, cogs_cents), fct_subscription_period (account_id, plan_tier, term_start_date, term_end_date) and dim_account (account_id, deployment_model, is_internal, effective_from, effective_to). Produce an exact decomposition of the five-point move into mix, rate and interaction terms, and name the segment shift that carries it.

Approach

Write blended margin as a revenue-weighted average of segment margins, m = sum over i of w_i * m_i, where w_i is the segment's share of net_amount_cents. Margin is a ratio of sums, so only revenue weights reproduce the blended figure.
Apply the exact three-term decomposition: delta_m = sum(delta_w_i * m_i0) + sum(w_i0 * delta_m_i) + sum(delta_w_i * delta_m_i), read as mix, rate and interaction. It is exact by construction, so the three terms must sum to the observed change with no residual.
Build segments from as-of joins: take the fct_subscription_period version whose term brackets each usage_date and the dim_account version whose effective_from and effective_to bracket it, rather than joining on is_current and backdating today's attributes over last year's usage.
Run the decomposition on more than one segmentation, at minimum plan_tier, deployment_model and sku_code, since the mix that actually moved may not be the one anybody already suspected.
Once the driving segment is identified, determine whether the shift came from acquisition landing new accounts in a lower-margin segment or from existing accounts migrating, because those have different owners and different remedies.

Follow-up

The interaction term is sometimes large. What does it mean operationally, and when would you prefer a log-mean decomposition that distributes it across the other two terms?
Revenue-weighted average margin is one construction. What changes if leadership wants margin weighted by account count, and which question does each version answer?

Suggested practice checklist

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.

Day 1 — Fix 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.

Day 2 — One 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.

Day 3 — Only 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.

Day 4 — One 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.

Day 5 — Your 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.

Day 6 — The 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.

Day 7 — Taper

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.

Getting an analysis acted on

Work that nobody used is a common and unflattering pattern in data careers, and interviewers probe for it. Have a story about an analysis that changed a decision, and be specific about how you got it in front of the person who could act. Also have one about work that went nowhere, with your reading of why.

Tell me about a time you had to explain a complex model to a non-technical stakeholder.

Medium · behavioural and stakeholder questions

Approach

Pick a story where you drove the decision, not one where you observed it.
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

What would you do differently if you ran that project again?
What did you decide not to do, and why?

Scope an open-ended request to predict account churn

Medium · scoping · churn modelling · operating point

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?

Report an underpowered consumption test to a non-technical executive

Medium · communication · uncertainty · experimentation

An account-randomised packaging change ran six weeks across 900 paying accounts. The effect on billable units per account per month is plus 4.1 percent, with a 95 percent interval from minus 3.2 to plus 11.8 after clustering standard errors at the account and applying the pre-registered winsorisation at the 99th percentile. An executive with no statistical background wants one number this week to decide a full rollout. Produce a three-sentence spoken answer, one chart, and an explicit recommendation of ship, stop or keep running, with the cost of each option stated.

Approach

The interviewer is probing whether you can be decision-useful without either hiding the uncertainty or hiding behind it. Start from the decision rather than the statistics: establish what the executive would do differently at plus 4 percent versus zero, because if the action is identical the interval does not matter.
Translate the interval into consequences in units the executive already reasons about. Multiply both endpoints by the cohort's baseline consumption and contracted rates to give an annualised revenue range, so the answer is a range of dollars rather than a range of percentages.
Price the option to wait. Using the observed variance, state roughly how many additional account-weeks halve the interval width, so keep running becomes a quantified choice instead of a stall.
Offer a cheaper path to the same decision: a lower-variance proximate outcome such as successful billable units on the new SKU, or CUPED using each account's pre-period consumption, quoting the expected variance reduction as one minus the squared pre-post correlation.
Give a recommendation and name the single observation that would reverse it. A strong answer commits; a generic one recites the interval and leaves the decision on the table.

Follow-up

The executive says it clearly works and is just not provable, so ship it. What is your answer?
How much of the interval width comes from clustering and how much from the revenue tail, and what would you do about each?
If you had to ship this week with no more data, which guardrail would you watch for the first fortnight and at what threshold would you roll back?