Glossary · Phase 11 · Professional practice
Primary metric
The one number an experiment is designed to move, chosen before it starts, which the launch decision hangs on.
Try it now
A guardrail stops a launch; a validity check stops you reading the results at all.
Learn it properly
A/B testing a model change · Hash assignment, per-user results from raw logs, sample ratio checks, guardrails and a decision rule. About 40 minutes.
Other terms from the same lesson
- guardrail metric: A number an experiment must not make worse, such as response time or cost; breaking its limit blocks a launch whatever the main result.
- novelty effect: Extra attention anything new gets in its first days, which fades and can make a change look better early on than it really is.
- peeking: Checking an experiment's result repeatedly and stopping at the first significant one, which makes false wins far more likely than the stated 5%.
- sample ratio mismatch: Group sizes in an experiment that are far from the planned split, a sign that something treated the groups differently, so no result can be trusted until it's explained.
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