Study preregistration has become increasingly popular in psychology, but its effectiveness in restricting potentially biasing researcher degrees of freedom remains unclear. We used an extensive protocol to assess the strictness of preregistrations and the consistency between preregistration and publications of 300 preregistered psychology studies. We found that preregistrations often lack methodological details and that undisclosed deviations from preregistered plans are frequent. Combining the strictness and consistency results highlights that biases due to researcher degrees of freedom are prevalent and likely in many preregistered studies. More comprehensive registration templates typically yielded stricter and hence better preregistrations. We did not find that effectiveness of preregistrations differed over time or between original and replication studies. Furthermore, we found that operationalizations of variables were generally more effectively preregistered than other study parts. Inconsistencies between preregistrations and published studies were mainly encountered for data collection procedures, statistical models, and exclusion criteria. Our results indicate that, to unlock the full potential of preregistration, researchers in psychology should aim to write stricter preregistrations, adhere to these preregistrations more faithfully, and more transparently report any deviations from the preregistrations. This could be facilitated by training and education to improve preregistration skills, as well as the development of more comprehensive templates.
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