A/B Testing

Running two versions of the same thing, ad creative, email subject line, product page, simultaneously to see which one performs better.

Assumptions are expensive. A/B testing replaces guessing with data. Even a 10% improvement in a key metric compounds significantly at scale.

You run two email campaigns: one with subject line "New arrivals are here" and one with "Your faves just dropped." The second gets 35% higher open rates. You use that style going forward.