The last two articles left us with two bad options: peek freely and inflate your false positive rate, or lock yourself into a single fixed-horizon look and lose all ability to react early — for a win or for a regression. Group Sequential Testing (GST) is the framework that gives you a third option: look several times, on a plan you commit to in advance, without breaking your error guarantees.
What Group Sequential Testing Actually Is
A group sequential test is any experiment design where you pre-specify a small number of interim analyses — "looks" — at planned points during data collection...