Seeing what you want to see in randomised controlled trials: versions and perversions of UKPDS data

doi:10.1136/bmj.320.7251.1720

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James McCormack, associate professor (jmccorma@interchange.ubc.ca)a,
Trisha Greenhalgh, senior lecturerb

^a Faculty of Pharmaceutical Sciences, University of British Columbia, Vancouver, Canada V6T 1W5
^b Department of Primary Care and Population Sciences, Royal Free and University College Medical School, London N19 3UA

Correspondence to: J McCormack

Accepted 28 March 2000

Randomised controlled trials are objective, free of bias, and produce robust conclusions about the benefits and risks of treatment, and clinicians should be trained to rely on them; so says the gospel of evidence based practice. In this article we argue, using the United Kingdom prospective diabetes study (UKPDS) as an example, that there is one stage in the conduct of a randomised controlled trial—the interpretation and dissemination ofresults—that is open to several biases that can seriously distort the conclusions. By bias, we mean the epidemiological definition: anything that systematically distorts the comparisons between groups. We will argue that certain biases arise when different stakeholders assign their individual values to the interpretation of the final results of randomised controlled trials.

Summary points

Randomised trials are subject to interpretation bias as shown by the example of the UK prospective diabetes study

The UK prospective diabetes study shows no benefit on macrovascular end points in patients with type 2 diabetes treated with sulphonylureas or insulin over 10 years

The study shows a clinically important benefit on macrovascular end points from metformin in patients with type 2 diabetes that seems somewhat independent of the drug's ability to lower blood glucose concentrations

Nevertheless, many authors, journal editors, and the wider scientific community interpreted the study as providing evidence of the benefit of intensive glucose control

Journal editors should be aware of this important potential bias and encourage authors to present their results initially with a minimum of discussion so as to invite a range of comments and perspectives from readers

Marketing the UK prospective diabetes study results

Until 1998, type 2 diabetes had been treated for over 25 years with drugs such as the sulphonylureas, insulin, and metformin. Only one well designed, prospective clinical trial had evaluated the effect of these drugs on the development of microvascular and macrovascular disease. This was the …