Evaluation, Testing & Optimization·Task 4.3·Bloom: evaluate·Difficulty 4/5·10 min read·Updated 2026-07-14

Diagnosing an Underpowered, Outcome-Shopped Experiment

Conduct A/B testing and iterative improvements

SUBy Solomon UdohReviewed by Solomon UdohAI-assisted · human-reviewed
In short
Diagnosing a flawed A/B comparison means recognising the compound failure of an undersized sample, an uncontrolled input distribution, and post-hoc metric selection. A small sample such as 50 sessions per arm can show a gap within the noise floor that disappears at scale; an uncontrolled distribution where one arm sees easier inputs can produce an apparent win that is really a sampling artifact; and choosing the primary metric after seeing which one moved converts the experiment into outcome-shopping. Any one of the three is sufficient to invalidate a result.

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