Outcome measures
Primary outcome was recruitment target not achieved within 6 months after the pre-planned recruitment period. These RCTs were defined as RCTs with recruitment failure. The pre-planned recruitment period was documented by the principal investigator before the start of the trial. Secondary outcomes included recruitment target not achieved within an extension period of at least 12 months and premature termination of the trial (defined as stopping with including patients before the recruitment target was achieved). All studies that recruited during the COVID-19 pandemic received 6 months extension of their recruitment period.
In all RCTs, we collected information on variables with a potential effect on recruitment failure, identified after a scoping review. We recorded variables at five levels; patient, doctor, participating centre, study organisation and study design (Appendix 1).