Table 1 Examples of studies reporting about recruitment problems in randomized controlled trials (RCTs)

Charlson et al.

1984

41 RCTs (≥ 250 patients) identified by an inventory of the National Institute of Health in 1979; investigator survey was principal data source

A third of RCTs recruited fewer than 75% of their planned sample size

Easterbrook et al.

1992

720 research protocols (N = 137 RCTs) approved by REC (UK); investigator survey was principal data source

Main reason (28%) for terminating the study was slow recruitment of patients

Wilson et al.

2000

RCT that investigated two management strategies for dyspepsia in primary care (UK)

90 primary care physicians were contacted; 43 agreed to participate, 31 recruited at least one patient, only 23 recruited more than 5 patients.

Foy et al

2003

7 primary care trials of dyspepsia management in the UK

One study reached its recruitment target; five recruited less than 50% of target and three of those closed prematurely

McDonald et al.

2006

114 RCTs funded by the Medical Research Council and Health Technology Assessment (UK); full scientific applications and subsequent trial reports were principal data source

Less than a third of the trials achieved their original recruitment target

Toerien et al.

2009

133 publications of RCTs identified by a systematic literature review (restricted to six major journals)

Of those trials reporting sample size calculation, 21% failed to achieve planned numbers at randomisation and 48% at outcome assessment.