Conversion rate

It is the clearest measure of how well a page, ad, or funnel step turns traffic into outcomes, independent of how much traffic there is. The one discipline it demands is a consistent denominator — sessions, users, and clicks each produce a different rate.

Conversion rate is only comparable within a consistent definition and a large enough sample. Small samples make rates noisy, which is why differences between variants should be checked for statistical significance before acting on them.