The predictive factor--a method to simplify Bayes' formula and its application to diagnostic procedures.

According to Bayes' rule the predictive value (PV) of a diagnostic test (= probability of disease if the test is positive) depends on the prevalence of the disease (= a priori probability), the sensitivity (c1) and the specificity (c2) of the test. A new variable has been introduced, the predictive factor (c), which is calculated as follows: c = c1/(c1 +1 -c2). Since the PV only depends on this factor and on the prevalence, the calculation is much easier and a general graphical solution is possible. This simplification renders several additional advantages and facilitates the understanding of the dependence of PV on prevalence.