A/B testing fundamentals, done rigorously

An A/B test is a chain of decisions made before you ever see a number. The interviewer is checking whether you make those decisions in the right order and can defend each one.

At Cursor the unit of analysis is the user-AI interaction, billions per period and the product is non-deterministic. That changes the texture of experiment design but not its skeleton. A rigorous test starts from a sharp hypothesis and walks down to a duration you can name out loud: one primary metric you are powered to move, the guardrails you refuse to harm, a randomization unit and an MDE that fixes your sample size.