Statistical analysis
For the primary outcome, we used the planned recruitment period as documented in the General Assessment and Registration form, a form that needs to be submitted to the ethical committee before actual start of the study. If we could not get access to this form, we retrieved this information from the main investigator and/or used the data mentioned in the protocol of the study. The actual recruitment period was calculated as the time between the first and last inclusion date.
We checked the continuous potential indicators with spline curve analysis. We dichotomised on basis of the spline curve and used the median when the spline suggested a straight line. We used logistic regression to evaluate the association between potential indicators of recruitment failure and expressed these as odds ratios (OR) with corresponding 95% confidence intervals (CI).
To further explore the most relevant risk factors for recruitment failure multivariable risk prediction modelling was done by using both forward and backward stepwise logistic regression (entry p=0.2 and exclusion p=0.1).
We used SPSS (IBM 2019, USA) software for all statistical analyses (version 25).