At the Psychological Methods Discussion group, Ben Ambridge asked the following question:
Hi everyone - I was wondering (don’t worry, I haven’t actually done this!) what would be wrong statistically speaking with an approach where you run a frequentist t-test (or whatever) after adding each participant and stop testing participants when the p value has remained below 0.05 (or 0.001 or whatever) for - say - each of the last 20 participants.