This is necessary with textbook statistical methods which are explicitly based on the assumption of looking at exactly one time. However, modern tools (Optimizely, VWO, Qubit) do not require this. [1]<p>If you use one of these tools it&#x27;s completely safe to run tests until the test says win.<p>There are still caveats to account for the real world - e.g., only stop the test after an integer number of weeks - but statistically this is a solved problem.<p>[1] The blog post under discussion was written before Optimizely&#x27;s StatsEngine and VWO&#x27;s SmartStats.