Unanswered questions and future research
Our findings suggest that more guidance is needed to ensure that external pilot trial protocol and results publications can be identified from the literature. Five respondents who were sent the appropriate survey for corresponding authors of external pilot trial protocols indicated that they had published their pilot trial results, yet we did not identify these publications. This highlights the difficulty in linking external pilot trial protocols to results publications, even when a systematic approach is used. Possible explanations for this are that the corresponding first author changed, that the external pilot trial title or acronym changed, or that the results publication was published in the time between the literature search and sending of the survey.Our findings also highlight the need for clearer guidance for progression criteria reporting. Some authors did not necessarily agree with our assessment of progression criteria reporting. Potential explanations for this are that authors had prespecified progression criteria, but this was not clearly reported in the publication, or that interpretations of progression criteria differ, which is perhaps likely given that there is no clear guidance for progression criteria. To address this, research is ongoing to develop guidance specific to the application and reporting of progression criteria in external randomised pilot trials.