Introduction: In common with other groups within the Cochrane Collaboration, the Diabetes Review Group is developing a specialised register of randomised controlled trials (RCTs) compiled by searching electronic databases and through systematic hand searching of the literature. The effectiveness of these combined methods in identifying all RCTs, and thus the completeness of the register, can be tested using a technique which examines the degree of overlap between these methods of ascertainment and uses a simple formula to estimate the size of the true population of trials.

Objective: To estimate the true number of RCTs reported in a given body of literature using the 'capture-recapture' method and estimate the proportion missed by the combination of MEDLINE and hand searching.

Methods: MEDLINE was searched to identify RCTs published over a 10 year period in the journal Diabetic Medicine. The journal was also hand searched for RCTs. The size of the 'total population' of publications dealing with RCTs was calculated using the 'capture-recapture' formula and the number of RCTs unidentified by either method was calculated.

Results: 115 RCTs were identified by both methods, 35 by the MEDLINE search alone, and 8 only by handsearching. An estimated 2.4 RCTs were missed by both.

Discussion: The use of 'capture-recapture' methodology demonstrates the effectiveness of the Diabetes Groups search strategy and provides an estimate of the completeness of its specialised register of RCTs.