Esimplicity Data Scientist Interview Questions & Guide 2026

The 60-second brief

As a Data Scientist II at Esimplicity, you play an integral role in harnessing the power of data to drive actionable insights and inform strategic decisions. This position is pivotal not only for the development of innovative products but also for enhancing user experiences and improving operational efficiencies. With the increasing complexity of data available today, your analytical skills will be essential in navigating and interpreting this information to benefit both the organization and its clients.

The boundary with engineering varies enough to be worth asking about directly. Some seats end at the analysis and the writeup; others carry feature pipelines, scheduling and alerting, which decides whether production data plumbing belongs in your prep at all.

Focus areas

• Decompose a metric move by segment and mix
• Size an experiment before anyone launches it
• Pick a randomisation unit that respects interference

Overview

In this role, you will engage with cross-functional teams, including engineering and product management, to solve real-world problems. You will be expected to work on diverse projects that span various domains, from predictive modeling to data visualization, impacting the company’s direction and market position. The work is challenging but rewarding, as you will contribute to projects that are both technically demanding and strategically significant.

The Esimplicity interview process

Esimplicity candidates report 4 rounds · ≈ 3-5 weeks. The stages below are what candidates describe, not a published process.

Initial Screening

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 Interviews

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

Case Study Discussion

A case has a fixed clock, and a good deal of what is being scored is how you spend it. Thirty to forty-five minutes buys one pass across the whole problem or a deep read of one part of it, and choosing between those is the work rather than a compromise forced on you. Announce the shape early: the structure you are using, the branch you think carries the decision, and what you are setting aside. An answer that is thorough for the first third and silent on the recommendation reads worse than one that is rougher throughout and lands.

What to demonstrate

• Whether a visible structure appears in the opening minutes and survives the rest of the case
• Whether the depth goes to the branch that carries the decision, rather than the branch you find most comfortable
• Whether you say what you are leaving out and why, instead of quietly omitting it and hoping nobody asks

How to prepare

• After each practice case, write down the branches you chose not to open and the reason for each, then check whether you said any of them out loud while the case was running. A branch you only cut privately reads to the interviewer as one you missed.
• Redo a case you have already worked in half the time, deciding in advance which single branch you keep, then compare which version a listener would find more useful.
• Write a two-sentence opening you can reuse, holding the restated question and your plan for the available time, and deliver it within the first ninety seconds of every practice run.

Behavioral Interviews

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.

Where candidates lose points

Watching an experiment daily and stopping when it crosses significance

A fixed-sample test controls type I error at one pre-declared look. Checking repeatedly and stopping at the first p < 0.05 inflates the false positive rate to roughly 0.15 to 0.20 for ten looks, and it rises further with more frequent checks, because the p-value takes a random walk that will eventually dip below the threshold under the null. The usual defences are a fixed horizon declared before launch, group-sequential boundaries such as O'Brien-Fleming that spend alpha across a planned number of looks, or always-valid confidence sequences that are correct under continuous monitoring. Compounding it, the effect size reported conditional on having crossed the threshold is biased away from zero, and the bias is larger the lower the power was, so an underpowered test that 'won' typically overstates the lift it found.

Treating last-touch attribution as the causal value of a channel

The attribution label on dim_user is the output of a rule that assigns full credit to whichever touch happened to be recorded last inside a lookback window, and that rule systematically rewards channels that sit close to the conversion, especially branded search and retargeting, which largely intercept demand that already existed. Reallocating spend on those labels moves budget toward the channels that are best at being last, which is why attributed return on ad spend often improves while total signups do not. Nothing in the touchpoint data can settle this, because the counterfactual of not running the channel was never observed. The credible reads are a geo holdout or a scheduled pause, sized in advance on the total-signups metric rather than on the attributed one, and the honest framing in the meantime is that the label describes correlation with conversion and not incremental contribution.

Over-explaining the method and under-explaining the implication

Lead with the answer and what you would do about it, then give the approach when asked. Roughly one sentence of method per three of implication is the right ratio for a stakeholder-facing answer; the interviewer already knows what a regression is.

Interpreting a change before checking data quality and logging

Spend the first pass on row volume by day, null rates, duplicate keys, and whether the step change lands on a release or tracking-migration date. A discontinuity that coincides with a deploy is an instrumentation hypothesis before it is a behavioural one.

Questions & practice

Discuss the bias-variance tradeoff.

Medium · statistics and probability

Approach

1. Quantify uncertainty explicitly rather than reporting a point estimate alone.
2. Translate the result into the decision it informs, in one plain sentence.
3. Say what the estimate is of, and over what population it generalises.

Follow-up

• Which assumption here is most likely to be violated in practice?
• What sample size would you need to detect an effect half this size?

How would you implement a decision tree from scratch?

Medium · machine learning and modelling

Approach

1. Set a baseline first, so any model has something honest to beat.
2. Say how the offline result would be validated online before it is trusted.
3. Check what information would not exist at prediction time, and exclude it.

Follow-up

• How would you choose the decision threshold, and who owns that choice?
• What would you monitor after launch to know the model is still valid?

Given a dataset, how would you approach building a predictive model?

Medium · machine learning and modelling

Approach

1. Check what information would not exist at prediction time, and exclude it.
2. Pick an evaluation metric that matches the cost of each error type, not a default.
3. Frame the prediction: the label, the moment of prediction, and the action it triggers.

Follow-up

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

How would you prioritize features when developing a new model?

Medium · machine learning and modelling

Approach

1. Check what information would not exist at prediction time, and exclude it.
2. Say how the offline result would be validated online before it is trusted.
3. Set a baseline first, so any model has something honest to beat.

Follow-up

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

Sessionise an event stream with gap and midnight rules

Hard · sessionisation · vectorisation · event streams

Sessionise a raw event stream. Input: a DataFrame with visitor_id, user_id (often NULL), occurred_at_utc and event_name, unsorted, up to 5 million rows. A session breaks when the gap from that visitor's previous event exceeds 30 minutes, and is force-closed at UTC midnight so no session spans two calendar dates. A gap of exactly 30 minutes does not break. Emit one row per session with session_id, visitor_id, the user_id as of the last event in the session, started_at_utc, ended_at_utc, session_date, duration_seconds and event_count. Vectorise; do not loop per visitor.

Approach

1. Sort by ['visitor_id', 'occurred_at_utc', 'event_id'] once, then express the whole problem as one boolean vector: a row starts a new session when the visitor changed, or the gap exceeds 30 minutes, or the UTC date differs from the previous row's UTC date. Cumsum that vector and you have the session key.
2. Get the comparison direction right on the gap: the rule is strictly greater than 1800 seconds, so an event at exactly 1800 seconds continues the session. Write it as gap > pd.Timedelta(minutes=30), and make the tie a test case rather than an assumption.
3. Derive the midnight break from the date change, not from inserting synthetic boundary rows. A date change implies a break even when the gap is two seconds, which is precisely the force-close rule and is why the two conditions are ORed rather than one subsuming the other.
4. Aggregate with a single groupby on the session key: min and max of occurred_at_utc, size for event_count, and last for user_id, which is correct because the frame is already sorted so 'last' is the final event in the session. That is the identity-as-of-session-end rule.
5. Compute duration_seconds as (max - min).dt.total_seconds(), which makes a single-event session 0 seconds. Say so explicitly, because a downstream mean session duration is sensitive to whether single-event sessions are 0 or excluded.

Worked solution (35 min)

1. df = df.sort_values(['visitor_id','occurred_at_utc','event_id']).reset_index(drop=True).
2. new_visitor = df.visitor_id.ne(df.visitor_id.shift()); gap = df.occurred_at_utc.diff(); new_day = df.occurred_at_utc.dt.date.ne(df.occurred_at_utc.dt.date.shift()).
3. is_start = new_visitor | (gap > Timedelta(minutes=30)) | new_day; df['session_key'] = is_start.cumsum().
4. g = df.groupby('session_key'); out = g.agg(visitor_id=('visitor_id','first'), user_id=('user_id','last'), started_at_utc=('occurred_at_utc','min'), ended_at_utc=('occurred_at_utc','max'), event_count=('event_id','size')).
5. out['session_date'] = out.started_at_utc.dt.date; out['duration_seconds'] = (out.ended_at_utc - out.started_at_utc).dt.total_seconds(); assign session_id from the sorted index.

Expected result: One row per session where session_count equals is_start.sum(), event_count sums exactly to len(input), every session's started_at and ended_at share one UTC date, and duration_seconds is at least 0 and strictly less than 86400.

Verify it yourself

• out['event_count'].sum() equals len(df); a mismatch means rows were lost to a join or a dropna.
• started_at_utc.dt.date equals ended_at_utc.dt.date on every row.
• For a planted visitor with events at 12:00:00 and 12:30:00 exactly, one session is emitted; move the second to 12:30:01 and two are emitted.
• For a planted visitor with events at 23:59:50 and 00:00:10, two sessions are emitted on consecutive dates despite a 20-second gap.

Follow-up

• Sessions are used as the denominator of a conversion rate. How does moving the inactivity gap from 30 to 45 minutes move that rate, and in which direction?
• A visitor's clock is 40 minutes ahead, so their events arrive with future occurred_at values. What does your sessioniser do, and what would you rather it did?
• The same person signs up mid-session on mobile and continues on desktop. How many sessions and how many users does your output show, and is that the right answer?

Write a function to calculate the mean and median of a list of numbers.

Medium · sql and data manipulation

Approach

1. Say which table is the grain you start from, and join outward from it.
2. Check whether any join is one-to-many before aggregating, or the sums inflate.
3. 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 time complexity of your solution?

Medium · sql and data manipulation

Approach

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

Follow-up

• How would you verify this result without re-running the same query?
• What breaks if events arrive late or out of order?

Pair flow starts to completions within a thirty-minute bound

Medium · event-streams · conditional-aggregation · flow-funnel

fct_event holds event_id, flow_id, flow_instance_id, event_name, occurred_at_utc, surface, app_version. Using only rows where event_name is 'flow_started', 'flow_completed' or 'error_shown', compute the daily core-flow completion rate by surface and app_version. Numerator: distinct flow_instance_id whose flow_completed occurs within 30 minutes of its flow_started with no error_shown for the same instance in between. Denominator: distinct flow_instance_id with a flow_started that day. Report rows with a NULL flow_instance_id as a separate coverage count, not inside either side.

Approach

1. Collapse the stream to one row per flow_instance_id using conditional aggregation: MIN(occurred_at_utc) FILTER (WHERE event_name = 'flow_started') AS started, MIN(...) FILTER (WHERE event_name = 'flow_completed') AS completed, MIN(...) FILTER (WHERE event_name = 'error_shown') AS first_error. This makes the 'first' semantics explicit and avoids a self-join entirely.
2. Apply the predicates on that single row: completed IS NOT NULL AND completed <= started + interval '30 minutes' AND (first_error IS NULL OR first_error > completed). Note the last clause encodes 'in between' literally, so an error surfaced after a successful completion does not disqualify the attempt.
3. Bucket by started::date, surface and app_version. Splitting by app_version is not decoration: a completion-rate regression is nearly always confined to one client build, and a pooled daily rate hides it behind the installed base.
4. Count NULL flow_instance_id rows as a separate coverage figure. They cannot be attributed to an attempt at all, so they belong in neither numerator nor denominator, and their share tells you how much of the rate is unmeasurable.
5. For any roll-up to week or to all surfaces, re-sum the numerator and denominator rather than averaging the daily rates.

Worked solution (25 min)

1. Write the per-instance collapse CTE and confirm COUNT(*) equals COUNT(DISTINCT flow_instance_id).
2. Add the three predicates one at a time, recording how many instances each removes.
3. Aggregate by day, surface and app_version with FILTER on the clean-completion flag.
4. Compute the NULL-instance coverage count as a separate select and report it alongside.
5. Pick the worst day-surface-version cell and pull ten raw instances to confirm the disqualification reason.

Expected result: One row per (day, surface, app_version) with started, completed_clean and a rate bounded by 0 and 1, plus a scalar coverage count of events carrying a NULL flow_instance_id. Instances completing after 30 minutes appear in the denominator only.

Verify it yourself

• No rate exceeds 1.
• COUNT(*) equals COUNT(DISTINCT flow_instance_id) in the collapsed CTE.
• Total started across cells equals the count of distinct flow_instance_id values carrying a flow_started event in the window.
• Instances with an error strictly between start and completion are excluded from the numerator but present in the denominator.

Follow-up

• occurred_at_utc is client-supplied. What do you do with an instance whose completion timestamp precedes its start?
• flow_instance_id is missing on 4 percent of starts for one app_version. What can and cannot you conclude about that build?
• An instance starts at 23:55 and completes at 00:04. Which day owns it, and what does the alternative do to the daily series?

Explain the difference between supervised and unsupervised learning.

Medium

Approach

1. Work from the decision backwards to the evidence you would need.
2. Say what you would check first and why it is the highest-information step.
3. State your assumptions explicitly before working the problem.

Follow-up

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

Tell a novelty effect apart from a durable one

Medium · novelty effect · exposure cohorts · composition

A redesigned navigation shows a 6% lift in weekly core actions in calendar week one, 3% in week two and 1% in week three, on a continuously enrolling user-randomised experiment. Enrolment ran throughout, so later exposure cohorts are younger accounts. Using fct_experiment_exposure (unit_id, variant, first_exposed_at_utc), dim_user.account_created_at_utc and fct_event, determine whether this is novelty decay, a composition artefact, or a real effect that shrank, and state what evidence would settle it.

Approach

1. Separate calendar time from exposure time first. With continuous enrolment, calendar week two mixes units in their second week of exposure with units in their first, and the tenure mix changes daily, so a calendar trend is not evidence of decay.
2. Rebuild the readout as a triangle indexed on days since first_exposed_at_utc. For each exposure cohort compute the treatment effect at exposure-day k, then average across cohorts at fixed k using only cohorts that have reached k. Real decay shows as a falling curve in k; a composition artefact flattens out.
3. Use the one clean discriminator novelty offers: it requires a prior version to be novel against. Split on whether the unit existed before experiment start. Users who signed up afterwards never saw the old navigation, so a stable effect among them is not novelty, and an effect confined to pre-existing users is.
4. Check the primacy direction as well. An early negative effect that recovers is the mirror image and argues for shipping rather than against it, and it is invisible without the same exposure-indexed curve.
5. Say what would settle it. A long-run holdback, a few percent of traffic kept on control for eight weeks or more and read from exposure-week six onward. No three-week test can distinguish a decaying effect from one that has reached a lower plateau.

Worked solution (30 min)

1. Assign each exposed unit an exposure cohort from date(first_exposed_at_utc) and compute treated and control means per cohort per exposure-day k.
2. Average the per-cohort effects at each k only across cohorts that have observed day k, leaving later k blank rather than averaging over whoever has reached it.
3. Split the same triangle on pre-launch versus post-launch account_created_at_utc from dim_user.
4. Compare the curves: decay confined to the pre-launch split is novelty, decay in both is a real effect shrinking, and a flat exposure-time curve under a falling calendar curve is composition.
5. Recommend the holdback and state the earliest exposure-week at which a plateau claim becomes supportable.

Expected result: The decision rests on the exposure-indexed curve, not the calendar one. Novelty is indicated only when decay is present among units that existed before launch and absent among units created after it. Identical decay in both splits means a real effect shrinking; no decay in exposure time means the calendar trend was composition. A three-week test cannot separate decay from a plateau, and a long-run holdback can.

Verify it yourself

• Within a cohort, denominators must not fall as k grows. If they do, the population is being conditioned on survival rather than held fixed at exposure.
• Re-aggregating the exposure-indexed effects with the observed enrolment weights must reproduce the reported calendar-week effects, which proves the two views are the same data.
• Verify with a query that no unit in the post-launch split has a pre-launch session, otherwise that split is not clean and the discriminator is worthless.

Follow-up

• The holdback costs a few percent of traffic on the winning arm. How do you justify that, and when would you retire it?
• Weekly core actions per user is a ratio whose numerator and denominator live at different grains. Which standard error are you computing, and why is the naive one wrong?
• What would you conclude if the effect were 6%, 3%, 1% in exposure time as well, among post-launch signups only?

Gross revenue churn doubled with no cancellations behind it

Hard · billing-periods · fx · revenue-churn · decomposition

Gross monthly revenue churn computed from fct_subscription_period doubled from 1.8% to 3.6% in one month. The support queue shows no rise in cancellations and renewals look normal. You have fct_subscription_period with subscription_id, account_id, period_start_utc, period_end_utc, mrr_cents, mrr_cents_constant_fx, seats_billed, period_status, change_reason and canceled_at_utc, plus dim_account. Decompose the 1.8-point rise into named mechanisms, size each in points of the headline, and state the remainder you cannot explain.

Approach

1. Reconstruct the numerator row by row and group it by change_reason before arguing about causes. A mid-period plan or seat change closes the current period row and opens a new one, so any implementation that treats a closed row as lost revenue books upgrades, downgrades and seat changes as churn; the change_reason breakdown of the numerator makes that visible in one query.
2. Check the recognition timestamp. Churn belongs at period_end_utc, because revenue continues until the period ends, not at canceled_at_utc when the button was pressed. Then count period_end_utc rows per month across thirteen months: annual cohorts concentrate their period ends in the month twelve months after they were signed, so a spike that repeats in the same month last year is seasonality in the book, not an event this month.
3. Recompute the whole numerator and denominator on mrr_cents_constant_fx. If the constant-currency figure is materially flatter, the move is an exchange-rate translation and belongs nowhere in a churn narrative.
4. Check period_status handling. Rows with period_status = 'past_due' are dunning, not cancellation; a status-based rule counts them as loss while a period-end rule does not, and a dunning backlog can move the number by itself.
5. Express every mechanism in points of the headline, sum them, and print the residual explicitly next to the month-to-month standard deviation of the prior twelve months. A decomposition without a stated remainder is a story, not an accounting.

Follow-up

• Which of these mechanisms should be fixed in the metric definition and which should be reported as a genuine business fact?
• How would you present a month whose churn is dominated by an annual cohort anniversary without the audience concluding the business is deteriorating?
• What would you change so that an upgrade can never enter the churn numerator again, and how would you test that it worked?

Suggested practice checklist

Four days spend equal time on query work, statistics, modelling and product judgement at deliberately shallow depth, which produces a scored map of where you actually stand. The last three days spend everything on the two areas the role weights most, and close by re-running day one to measure movement.

Day 1 — Breadth pass: query fluency

• Solve six prompts spanning aggregation, joins, window functions and date arithmetic in 60 minutes total, stopping at 10 minutes each whether or not it works, and mark every prompt as solved, solved slowly, or stuck.
• For each unsolved prompt write the single blocking sentence (I lost the grain, I did not know the frame clause, I could not express the date boundary) instead of reading the solution.
• Translate one pandas transformation you know well into SQL and one SQL query into pandas, checking that both return the same row count and the same totals.

Deliverable: A scored six-row table, one line per prompt, saved for the day-seven re-run.

Day 2 — Breadth pass: statistics and inference

• Answer ten short questions in writing with nothing open: what a p-value is conditional on, what a 95 percent interval covers across repeated samples, when a paired test is the right one, what the bootstrap estimates, why multiple comparisons inflate false positives, how controlling the family-wise error rate differs from controlling the false discovery rate, what power depends on, what a missed real effect costs a product, the three situations where the central limit theorem does not rescue you (small n, very heavy tails, dependent observations), and what a standard error is the standard deviation of.
• Grade yourself against a reference and count only the answers that were exactly right, not the ones that were nearly right.
• Rewrite the two weakest answers the following morning from memory in full sentences.

Deliverable: Ten graded answers with an honest count of exact hits.

Day 3 — Breadth pass: modelling

• Take one tabular dataset end to end in 90 minutes: a leakage-safe split, a baseline that is not a model (majority class or historical mean), one regularized linear model, one gradient-boosted tree, and a single evaluation metric chosen before you look at any result.
• Write why that metric fits the cost structure: precision at a fixed recall for alerting, calibration for anything feeding a price or a threshold, ranking metrics for retrieval, and note that area under the ROC curve is insensitive to class balance in a way that can flatter a rare-positive problem.
• Name the leak you were most likely to introduce (an encoding fit on all rows before splitting, or a feature computed after the label's timestamp) and write the check that would have caught it.

Deliverable: A notebook whose first cell states the metric and the baseline, plus two lines on what beat what and by how much.

Day 4 — Breadth pass: product judgement

• Answer three case prompts aloud at 15 minutes each, timing how long passes before you state a success metric.
• For one case write the first segmentation you would run and the row counts you expect per segment, so that a tiny segment cannot quietly drive the conclusion.
• Take a metric definition you did not write, from a public dashboard, a textbook, or documentation you already have open, and list every place two analysts implementing it would diverge: which rows the denominator admits, whether the unit is an account or a person, what the time window is anchored to, and what happens to data that arrives late. Then write the one question that would close the largest of those gaps.

Deliverable: Three recorded case answers plus an ambiguity list for a metric someone else defined, ending in the single question you would ask about it.

Day 5 — Depth, first area

• Rank the four areas by how many bullet points in the role description each one covers, pick the top one, and spend the entire day inside it.
• Work the six hardest problems you can find in that area and for each write the generalizable move you should have reached for first, rather than the answer.
• Re-solve the two you failed the same evening with notes closed.

Deliverable: Six generalizable moves written as instructions to yourself, not as solutions.

Day 6 — Depth, second area, and the seam between them

• Repeat the depth protocol on the second-ranked area with the same six-problem structure.
• Construct one problem that requires both areas at once, for example a metric redefinition whose effect you must validate with a test whose readout you then have to query.
• Solve your own combined problem end to end and note where the handoff between the two areas cost you time.

Deliverable: One combined problem, solved end to end, with the handoff failure written down.

Day 7 — Integration and re-measurement

• Re-run the six prompts from day one under the same clock and compare both correctness and time.
• Run a 60-minute mixed mock that moves between areas without warning, since switching cost is what breadth passes do not train.
• Write the two areas you would still fail on, and the sentence you will use in the interview when you hit one of them.

Deliverable: A before-and-after score table plus a written plan for the two remaining gaps.

What the behavioural round is actually scoring

Interviewers here are not checking whether you can describe a project. They want the decision you made, why you made it under the information you had, and what changed afterwards that someone else could measure. A story that ends at 'I built a model' has no ending. Say what the model caused, or what you stopped doing because of it.

Describe a time when you had to make a data-driven decision with limited information.

Medium · behavioural and stakeholder questions

Approach

1. Name the disagreement or constraint, and how you resolved it with evidence.
2. State the situation in two sentences and spend the rest on your reasoning.
3. Pick a story where you drove the decision, not one where you observed it.

Follow-up

• How did you know the outcome was caused by your change?
• What did you decide not to do, and why?

Describe a situation where you had to influence stakeholders.

Medium · behavioural and stakeholder questions

Approach

1. Quantify the outcome, including what you would not claim credit for.
2. Pick a story where you drove the decision, not one where you observed it.
3. Close with what you would do differently, concretely.

Follow-up

• What would you do differently if you ran that project again?
• How did you know the outcome was caused by your change?

Turn an ambiguous onboarding question into a measurable metric

Easy · scoping · metric definition · stakeholder

Two days before a planning review, a director asks whether onboarding is working. You have dim_user (account_created_at_utc, signup_surface, is_internal), fct_event (is_core_action, flow_id, flow_instance_id, event_name, occurred_at_utc, received_at_utc) and fct_session. No further meeting with the director is possible before you start work. Deliver three clarifying questions you would send in writing, the metric you will compute in the meantime with its numerator, denominator, window and exclusions, and one sentence naming the question you are deliberately not answering.

Approach

1. Recognise what is being probed: whether you convert a goal into a computable predicate without stalling for requirements or guessing in silence. Listing clarifying questions is the generic answer; shipping a defensible default alongside them is the strong one, because the review is in two days and it will happen with or without you.
2. Infer the decision behind the request. A question about whether onboarding works, arriving before a planning cycle, usually means whether to staff it next quarter. That points at a rate with visible headroom over several cohorts, not at a descriptive dashboard.
3. Write the three questions so that each one changes the SQL. Which population, all signups or only self-serve from dim_user.signup_surface. What counts as working, reaching a core action or completing the onboarding flow_id. Against what bar, last quarter's cohorts or a stated target.
4. Propose the default explicitly: seven-day activation on weekly signup cohorts. Numerator, users with is_core_action = TRUE events on at least two distinct UTC dates inside [account_created_at_utc, account_created_at_utc + 7 days). Denominator, the signup cohort with is_internal = FALSE. Publish with an eight-day lag, and state that the two-distinct-days threshold is a frozen choice rather than a discovery.
5. Name the exclusion in the same breath as the number. The series shows whether users activate; it does not establish that onboarding caused the level, which needs a staged rollout or an experiment.

Follow-up

• The director replies that they meant the onboarding flow specifically, not activation. What changes in the query and in the caveats?
• Your cohort metric needs an eight-day lag and the review is in two days. What do you present, and how do you label it?
• Two of your three questions come back unanswered. Which one do you refuse to proceed without?