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Your A/B test shows the control and treatment groups differ before the treatment even applies. What's wrong?

A pre-existing gap between your groups means randomization or instrumentation is broken, and the whole experiment is suspect. Strong candidates name sample-ratio mismatch on sight. Here is the full diagnosis and the disciplined response.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.

A pre-existing gap between your groups means randomization or instrumentation is broken, and the whole experiment is suspect. Strong candidates name sample-ratio mismatch on sight. Here is the full diagnosis and the disciplined response.

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