Optional Stopping

Definition: The practice of (repeatedly) analyzing data during the data collection process and deciding to stop data collection if a statistical criterion (e.g. p-value, or bayes factor) reaches a specified threshold. If appropriate methodological precautions are taken to control the type 1 error rate, this can be an efficient analysis procedure (e.g. Lakens, 2014). However, without transparent reporting or appropriate error control the type 1 error can increase greatly and optional stopping could be considered a Questionable Research Practice (QRP) or a form of p-hacking.