Randomized controlled trials in "Contraception": the need for "CONSORT" guidelines

The Cochrane Fertility Regulation Group: synthesizing the best evidence about family planning

The Fertility Regulation Group of the Cochrane Collaboration has been assessing the best available evidence on fertility regulation, family size and birth spacing. By the end of 2005, this group had published 32 systematic reviews and 12 protocols; most reviews were on contraception. Because of suboptimal trial quality, firm conclusions could be made in only five reviews. Threats to internal validity in published trials include the absence of description of allocation concealment, intentional exclusion of participants after randomization, failure to use intention-to-treat analyses and lack of treatment blinding. The precision of results has been limited by small sample sizes. The finding that most trials of oral contraceptives were conducted by pharmaceutical companies raises concerns about potential commercial bias. Of necessity, most information about fertility regulation effectiveness and adverse effects comes from observational studies, which vary widely in quality. Systematic reviews of evidence, with an emphasis on randomized controlled trials when available, will continue to improve fertility regulation in the years ahead.

Ensuring the comparability of comparison groups: is randomization enough?

BACKGROUND

It is widely believed that baseline imbalances in randomized trials must necessarily be random. In fact, there is a type of selection bias that can cause substantial, systematic and reproducible baseline imbalances of prognostic covariates even in properly randomized trials. It is possible, given complete data, to quantify both the susceptibility of a given trial to this type of selection bias and the extent to which selection bias appears to have caused either observable or unobservable baseline imbalances. Yet, in articles reporting on randomized trials, it is uncommon to find either these assessments or the information that would enable a reader to conduct them. Nevertheless, there have been a few published reports that contain descriptions of either this type of selection bias or indicators that it may have occurred.

OBJECTIVE

To document that the same type of selection bias has been described in numerous randomized trials and therefore that it represents a problem deserving of greater attention.

STUDY SELECTION

Computerized searches were not useful in locating trials with one or more elements that contribute to or are indicative of selection bias in randomized trials. We limit our treatment to trials that were previously questioned for susceptibility to selection bias or for large baseline imbalances.

RESULTS

We found 14 randomized trials that appear to be suspicious for selection bias. This may represent only the tip of the iceberg, because the status of other trials is inconclusive.

CONCLUSIONS

Authors of clinical trial reports should be required to disclose sufficient details to allow for an assessment of both allocation concealment and selection bias. The extent to which a randomized study was susceptible to selection bias should be considered in determining the relative contribution it makes to any subsequent meta-analysis, policy or decision.