Interview prep
Data Scientist interview questions
Data science interviews test statistical reasoning, SQL proficiency, and ability to frame business problems as data problems. Expect a mix of coding (Python), statistics, and case studies analyzing real datasets.
🎤 Practice these out loud with AceCoach →The questions
Walk me through your approach to an A/B test where the control group unexpectedly outperforms the variant in early days. Case
How to answerState the null hypothesis, describe sample size and duration logic, explain why reversal happens (novelty effect, selection bias, or luck), outline statistical checks before stopping early.
Write a SQL query to find users who made a purchase in January but not in February of the same year. Coding
How to answerSet up LEFT JOIN or NOT EXISTS with date filtering, verify the join logic, test edge cases (null purchase dates, multiple purchases).
Explain how you'd measure the impact of a recommendation engine change using holdout groups. Fundamentals
How to answerDefine the metric upfront (engagement, revenue, retention), explain why holdout is necessary, discuss sample size, duration, and external factors.
Tell me about a time you discovered a bug in a data pipeline and how you fixed it. Behavioral
How to answerSTAR format: Situation (pipeline scope), Task (what went wrong), Action (investigation steps, root cause), Result (fix applied, time saved, stakeholder communication).
How would you handle a dataset with 60% missing values in a key feature? Fundamentals
How to answerAssess missingness type (MCAR, MAR, MNAR), justify imputation (mean, KNN, model-based) or removal, discuss bias trade-offs, validate impact on downstream models.
Design a metric to detect when user churn risk is increasing week-over-week. System design
How to answerDefine early signals (engagement drop, feature usage decline), explain aggregation and thresholds, describe alerting cadence, outline how business would act on the signal.
What data scientists are tested on
PythonSQLStatisticsA/B TestingPandas/NumPyData VisualizationMachine LearningBusiness Acumen
Frameworks are guidance, not scripts — the point is to make the answers your own. All roles →