Stopping tests too early?

Stopping tests early based on interim results is one of the most damaging testing errors:

The statistical problem: Each time you check results and consider stopping, you effectively run a new test. Multiple tests inflate false positive rates far beyond your target confidence level.

How bad is it? Checking daily and stopping when results look good can produce false positive rates of 30% or higher, even when targeting 5%.

Why it feels right: When one variant is ahead by a large margin early, it seems wasteful to continue. But early leads frequently reverse or shrink as more data accumulates.

Prevention:

Calculate required sample size before starting. Commit to running to completion regardless of interim results. If you must peek, use sequential testing methods designed for interim analysis.

Patience produces truth. Early stopping produces comfortable lies.