Many randomised controlled trials compare treatments that will produce only moderate differences in outcome, but these differences can be clinically important. However, they are difficult to assess reliably and require a large amount of randomised evidence. This can be achieved through large prospective randomised trials which will accrue future patients, the meta-analysis of results from randomised trials involving patients from the past, or - ideally - both. The techniques require that all possible biases are minimised, and in meta-analyses this can best be achieved by ensuring that all of the randomised evidence - both trials and participants in those trials - is included. The meta-analysis of individual patient data has been described as the gold standard for this approach. It will remove many of the problems associated with relying solely on published data and some of the problems arising from a reliance on aggregate data, and will also add to the analyses that can be performed. Such projects, however, require considerable time and effort.

* This paper was presented at a meeting of Systematic Reviews organised jointly by the BMJ and the UK Cochrane Centre and held in London in July 1993

The differences in outcome between many of the treatments compared in randomised trials are moderate but potentially very important to patients and their medical carers. Individually, however, most trials have been too small to assess such differences reliably. There are two main ways to overcome this: through large prospective randomised trials which will accrue future patients, and through meta-analysis of completed trials. Whether a single randomised trial or a meta-analysis is to be undertaken, all possible biases should be minimised, and perhaps the most important step in this is to ensure that as much as possible of the randomised evidence is included in the analyses.1

This paper …