Pipelines, not models. Expect idempotency, late-arriving data, backfills and schema evolution — and at least one question about what happens when a job silently produces wrong numbers for a week.
Data loops test two things that pull against each other: rigour with methods, and the judgement to know when a rough answer is the right one. Candidates who are strong on one are often visibly weak on the other.
Expect questions where the data is deliberately insufficient. Stating the assumption you are making, and what would change your answer, is most of the mark.
Each round is run by the interviewer built for it, with its own scoring axes — not one generic interviewer asked to change subject.
Metrics, experiments, causal reasoning, and decisions under uncertainty.
Scored on Analytical communication · Data & inference rigor · Decision quality
Metrics, experiments, causal reasoning, and decisions under uncertainty.
Scored on Analytical communication · Data & inference rigor · Decision quality
Hands-on problem solving, code quality, debugging, and engineering judgment.
Scored on Problem-solving communication · Engineering correctness · Adaptability & craft
System design, trade-offs, and war stories from production.
Scored on Technical communication · Engineering depth · Ownership & collaboration
Metrics, experiments, causal reasoning, and decisions under uncertainty.
Scored on Analytical communication · Data & inference rigor · Decision quality
Six you can expect, in the register interviewers actually use. Answer them out loud before you read the next section — reading a question and answering one are different skills, and only the second is marked.
The gap between these two is most of your score, and it is easier to see than to be told.
“A daily job has been silently producing wrong numbers for a week. What happens?”
Fixing the job. It addresses the cause and leaves a week of bad numbers sitting in every downstream dashboard and model.
Stop the spread, quantify the blast radius, tell the consumers before they find out, then backfill, then add the check that would have caught it on day one. Silence is the defining word in the question and the answer has to deal with it.
The post-mortem nobody sends you. These are specific to this loop rather than general interview advice.
For Data Engineer, a general round runs with Marcus — metrics, experiments, causal reasoning, and decisions under uncertainty. It is the fastest way to find out which round you actually need to work on. The first session is free.
Practise a Data Engineer interview →Before you practise, it is worth reading how the common rounds are marked: tell me about yourself, behavioral questions and STAR, and system design.