Running Multiple A/B Tests at Once: When Overlap Is Safe

Every experimentation program hits this question within its first few months: test A is running on the homepage, the checkout team wants to launch test B — do we wait? Run them together? Will one contaminate the other? Teams that answer "always isolate" grind their velocity to a halt. Teams that answer "never worry" eventually ship a result that was quietly distorted by a colliding experiment. The correct answer is more useful than either extreme: overlap is usually fine, the risky cases are identifiable in advance, and when you do need isolation, Optimizely gives you exclusion groups to enforce it deterministically. This guide covers when overlapping A/B tests are safe, when they are not, and exactly how to configure mutually exclusive experiments in both Optimizely Web Experimentation and Feature Experimentation. The Short Answer: Overlap Is Usually Safe By default, Optimizely lets experiments overlap — a single visitor can be in test A and test B at the same time. B