Potential indicators of recruitment failure
The association of the potential indicators with RCTs with recruitment failure i.e. RCTs that did not achieve their recruitment target within the pre-planned study period with a maximal extension period of 6 months, is shown in Table 3.
Indicators associated with higher chances on recruitment failure were presence of a no-treatment arm, having a design with more than two arms, funding, a compensation fee of less than 200 euros per included patient, funding of less than 350.000 euros and having more than four inclusion criteria. An indicator associated with lower chances on recruitment failure was a preceding pilot study. The most relevant indicators for recruitment failure in multivariable risk prediction modelling were presence of a no-treatment arm (OR 4.95, 95% CI 1.18 to 20.80), a compensation fee of less than 200 euros per included patient (OR 2.90, 95% CI 1.02 to 8.25)), funding of less than 350.000 euros (OR 2.99, 95% CI 1.05 to 8.51), while a preceding pilot study lowered the risk for treatment failure (OR 0.21, 95% CI 0.05 to 0.83).
When we compared the 41 RCTs that did not achieve their recruitment target within the pre-planned study period with a maximal extension period of 12 months, with the 42 RCTs that completed recruitment within that period, the described associations with treatment failure remained comparable in direction and size.
The most relevant indicators for stopping prematurely were the absence of a preceding pilot study and having a no-treatment arm. None of the 19 RCTs that stopped prematurely had performed a pilot study (0%), compared to 17 of the 62 RCTs that completed recruitment (27%). Ten of the 19 RCTs that stopped prematurely had a no-treatment arm (52%), compared to eight of the 64 RCTs that completed recruitment (12.5%) (OR 6.13, 95% CI 1.98 to 19.06).