How Sequential Testing Lets You Stop A/B Tests Early Without Skewing Results

Standard A/B testing requires waiting for a pre-set sample size before checking results, but in practice teams often peek at dashboards early and stop tests when they see significance — a habit that silently inflates false-positive rates well beyond the intended 5%. This problem, known as 'peeking,' is not a matter of discipline but a statistical flaw rooted in how fixed-sample tests are designed. Abraham Wald's Sequential Probability Ratio Test (SPRT), originally developed for wartime quality control, offers a mathematically rigorous alternative that allows repeated checks without compromising error-rate control. Unlike peeking, the SPRT builds its stopping rule into the methodology from the outset, meaning early termination carries no statistical penalty. Sequential testing is particularly suited to situations where traffic is limited, testing windows are costly, or teams cannot commit to a fixed timeline — making it a different form of rigor rather than a shortcut.