What proportion of healthcare is evidence-based? Back in the mid-1970s, one of the great names of epidemiology, Kerr White, suggested in conversation with Archie Cochrane that 'only about 15-20 per cent of physicians' interventions were supported by objective evidence that they did more good than harm'.

'Kerr, you're a damned liar, ' interrupted Professor Cochrane, who had then recently stood down as head of the Medical Research Council's epidemiology research unit and was subsequently to be the inspiration for the work of the Cochrane Collaboration.'You know it isn't more than 10 per cent.'

However evidence-based their own assessments, and to the chagrin of many doctors, the figures have stuck ever since.

Indeed, Professor White, deputy director of health sciences at the Rockerfeller Institute, claimed later the estimate was sound, but he had inflated it to avoid 'startling' clinicians in the audience.

The anecdote comes from a useful resource guide, produced by Andrew Booth of Sheffield University, which seeks to answer the question at the top of this column. As you might expect, it is not possible to come up with a definitive percentage. It depends what you mean by evidence, or how you define the intervention.

A 1995 study at Oxford Radcliffe Hospital found that 82 per cent of interventions in general medicine were based on high-quality scientific evidence. The finding prompted one doctor to write to The Lancet condemning evidence- based medicine as 'an example of newspeak [which] would have delighted George Orwell'.

Ofcourse, the current debate is not whether doctors are basing their work on good evidence, but on whether doctors are themselves any good. A growing consensus seems to be that somewhere between a tenth and a third of the medical profession is not up to it. There's one for the Commission for Health Improvement to ponder.

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