Once you have seen a result, it is surprisingly easy to tell a convincing story about why it was expected. Preregistration preserves an earlier record. Researchers specify what they plan to test and how before the relevant outcomes are known.
The distinction between planned and exploratory analysis helps readers interpret evidence. A pattern discovered while searching the data can be valuable, but it has a different status from a successful test of a previously specified prediction. A replication can then examine a finding with new observations.
Leaving room for discovery
Suppose a team plans to compare two ways of recruiting volunteers. It could record the main outcome, who will be included, and how the comparison will be analyzed. If the results later suggest an unexpected difference between neighborhoods, the team can investigate it while making clear that this was a new question.
Preregistration does not freeze thought or repair a poor design. Plans may need to change because an instrument fails or circumstances shift. Explaining the departure preserves the value of the record better than pretending the original plan remained untouched.
An inspectable commitment
Specificity matters. A vague promise to analyze engagement leaves many choices open. Conversely, a detailed plan nobody can locate or compare with the final report does little for accountability. The social mechanism is an inspectable commitment: it gives other people a way to distinguish what the research tested from what it discovered along the way.