Defining Success: Metrics and Guardrails

You can't improve what you don't measure. Before launching any experiment, you need to answer a deceptively simple question: What does success look like?

This sounds obvious, but it's where many teams go wrong. They launch tests without clear success criteria, then argue about what the results mean. Or worse, they optimize for a metric that looks good in isolation but hurts the business overall.

A well-designed experiment needs two types of metrics: a success metric that tells you whether to ship, and guardrail metrics that ensure you're not winning at the cost of something else important. In this section, you'll learn how to choose both—and why the distinction matters.