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).