When a physician suspects that a patient has a disease and the laboratory examination or clinical test ordered for this purpose comes back "positive," this does not necessarily mean that the patient has the disease. Everyone is familiar with the notion of a "false positive." The physician must therefore use the positive test result and the other available information to assess correctly the patient's chances—or probability—of having the suspected disease.
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A little theory
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As a practical illustration, we shall consider measuring blood glucose (sugar) levels with a view to a possible diagnosis of diabetes, a rapidly spreading disease whose devastating effects on health are well known. But first, we need to review some theoretical concepts that recur when modeling situations of this kind.
Let M, M, T and T respectively denote the following events:
