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What is CUPED, why does it shrink experiment variance, and how does it compare to stratification and regression adjustment?

CUPED can cut the sample size an A/B test needs by half without touching validity. The signal is explaining why subtracting a pre-experiment covariate reduces variance but never biases the estimate. Here is the answer.

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

CUPED can cut the sample size an A/B test needs by half without touching validity. The signal is explaining why subtracting a pre-experiment covariate reduces variance but never biases the estimate. Here is the answer.

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