Can I stop a test early if results look clear?

Stopping tests early based on apparently clear results is statistically dangerous and inflates false positive rates dramatically.

Peeking problems: When you check results repeatedly and stop when they look good, you are essentially running multiple tests. Each peek increases the chance of catching a random fluctuation that looks significant.

Regression to the mean: Early leads often shrink or reverse as more data accumulates. What looks like a clear winner at 1,000 observations may be a tie at 10,000.

If you must peek, use sequential testing methods designed for interim analysis, such as group sequential designs or Bayesian approaches that properly account for multiple looks.

The safest approach is determining sample size beforehand and committing to run until completion regardless of interim results.

Early stopping feels efficient but produces unreliable results. The time saved is not worth acting on potentially false conclusions.