Evidence-appraisal glossary
Stopping rule
A stopping rule is a pre-specified statistical threshold that determines when a trial may be ended early, whether for clear benefit, harm, or little chance of a useful answer.
Also called: stopping boundary, early stopping rule.
Because looking at accumulating data repeatedly inflates the chance of a false positive, stopping rules set stricter thresholds at interim looks to keep the overall error rate controlled. Trials halted early for benefit tend to overestimate the true effect, since they stop on a random high point, so an impressive result from an early-terminated trial deserves extra caution. The rule should be defined in the protocol before any interim data are examined.
Read the full Reading the Evidence blog.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.