Metrics That Matter

A primary metric is the one number the test is allowed to move. Click-through rate is the most common, and the most misleading. Clicks that do not convert are vanity.

Choose your primary metric as close to revenue as your sample size allows. On low-traffic surfaces, you may have to use CTR as a leading indicator — but always declare a downstream guardrail metric that must not regress.

A guardrail metric protects you from optimising a local maximum at the expense of the whole funnel. If your hero CTA test lifts CTR 8% but drops checkout completion 4%, the test loses. Always.

const test = {
  primary: 'hero_cta_clicks / hero_views',
  guardrails: ['checkout_completion_rate', 'revenue_per_visitor'],
};