How to Set a Minimum Detectable Effect That Actually Works for A/B Tests
When setting up an A/B test, the Minimum Detectable Effect (MDE) is the smallest true improvement a test is designed to reliably identify, and it directly determines how much traffic and time an experiment requires. Unlike other inputs in a sample-size calculator, MDE has no default correct value, making it a business judgement as much as a statistical one. A common source of confusion is that most calculators express MDE as a relative lift rather than an absolute percentage-point change, meaning a 10% MDE on a 20% baseline conversion rate targets a 2 percentage-point shift, not 10 points. Setting the MDE too small can leave a test running indefinitely, while setting it too large risks missing meaningful improvements the business actually cares about. Understanding the relationship between MDE, baseline conversion rate, and sample size is essential to designing experiments that are both feasible and informative.
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