Test-subject objections

November 15, 2012

So the University of Aberdeen has developed a test for schizophrenia that is 98 per cent accurate (Campus round-up, 8 November). Should I be worried if I test positive?

Around 1 per cent of the population has schizophrenia, so in a sample of 10,000 you will get 98 of the 100 with schizophrenia correctly testing positive. However, 2 per cent of the 9,900 - 198 - who haven't got it will also test positive. Therefore, the probability of a positive test being correct is 98/(98+198), or 33 per cent. So, more than two-thirds of those positively diagnosed by the test will in fact not be schizophrenic. Reassuringly high accuracy rates are meaningless unless one knows the prevalence of what is being tested.

Hillary J. Shaw, Senior lecturer, Department of food and supply chain management, Harper Adams University College

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