The view is widely held that experimental methods (randomised controlled trials) are the “gold standard” for evaluation and that observational methods (cohort and case control studies) have little or no value. This ignores the limitations of randomised trials, which may prove unnecessary, inappropriate, impossible, or inadequate. Many of the problems of conducting randomised trials could often, in theory, be overcome, but the practical implications for researchers and funding bodies mean that this is often not possible. The false conflict between those who advocate randomised trials in all situations and those who believe observational data provide sufficient evidence needs to be replaced with mutual recognition of the complementary roles of the two approaches. Researchers should be united in their quest for scientific rigour in evaluation, regardless of the method used.

Despite the essential role of observational methods in shedding light on the effectiveness of many aspects of health care, some scientists believe such methods have little or even nothing to contribute. In his summing up at a major conference held in 1993, the eminent medical epidemiologist Richard Doll concluded that observational methods “provide no useful means of assessing the value of a therapy.”1 The widely held view that experimental methods (randomised controlled trials) are the “gold standard” for evaluation has led to the denigration of non-experimental methods, to the extent that research funding bodies and journal editors automatically reject them. I suggest that such attitudes limit our potential to evaluate health care and hence to improve the scientific basis of how to treat individuals and how to organise services.

My main contention is that those who are opposed to the use of observational methods have assumed that they represent an alternative to experimentation rather than a set of complementary approaches. This in turn stems from a misguided notion that everything …