- D G Altman,
- J M Bland
- Medical Statistics Laboratory, Imperial Cancer Research Fund, London WC2A 3PX
- Department of Public Health Sciences, St George's Hospital Medical School, London SW17 1RE.
We have previously considered diagnosis based on tests that give a yes or no answer.1, 2 Many diagnostic tests, however, are quantitative, notably in clinical chemistry. The same statistical approach can be used only if we can select a cut off point to distinguish “normal” from “abnormal,” which is not a trivial problem. Firstly, we can investigate to what extent the test results differ among people who do or do not have the diagnosis of interest. The receiver operating characteristic (ROC) plot is one way to do this. These plots were developed in the 1950s for evaluating radar signal detection. Only recently have they become commonly used in medicine.
We assume that high values are more likely …