Author's reply

BMJ 1995; 311 doi: https://doi.org/10.1136/bmj.311.7014.1231 (Published 04 November 1995) Cite this as: BMJ 1995;311:1231
  1. Bob Ryder
  1. Consultant physician Diabetic Unit, City Hospital, Birmingham B18 7QH

    EDITOR,--I would like to illustrate the problem with Wallace C Foulds and Angus C MacCuish's interpretation of their data by using the following analogy. Suppose the incidence of phenylketonuria is 1 in 10000 births. If a screening programme picked up 40 cases out of the 100 in a population of one million this would be 0.004% of the population, whereas the actual number of cases would be 0.01% of the population. The difference between these figures is 0.006%. However, the screening programme failed to detect 60 of the 100 cases. One would therefore have to say that the screening programme failed to detect 60% of the cases of …

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