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).