🟢 Stats: CUPED (Advanced — 2024-25 Interview Trend)

What it is: Controlled-experiment Using Pre-Existing Data. Reduces variance by adjusting for pre-experiment behavior.

The intuition: If a user was already high-spending BEFORE the experiment, their high spending DURING the experiment isn't due to the treatment. CUPED subtracts this pre-experiment baseline, reducing noise and letting you detect smaller effects with the same sample size.

Where it's used: Meta, Netflix, Microsoft, Uber — almost every major tech company.

Why interviewers ask: It shows you understand that A/B testing isn't just "compare two means." It's about reducing variance to make better decisions faster.