Involves looking at a test result before the test has concluded and deciding to stop the test because it looks like a winner (or loser) has emerged.

Peaking at a test result is like baking a cake, opening the oven and deciding the cake is ready before it’s fully finished.

Even though it’s tempting to stop the test early and dig into that delicious win, or prevent a loss, the results are only half-baked.

Before pulling a cake out of the oven, a good baker pokes a tooth pick, or fork, into the cake batter to make sure it’s truly ready. If the batter sticks to the fork, the cake needs to stay in the over longer longer.

The same goes for A/B testing.

In a split-test, you should calculate the required sample size ahead of time to know if you've received enough traffic to declare a winner or loser.

If the necessary sample size hasn't yet been reached, you know you need to keep the test running longer.