The positive and negative predictive values of a dignostic test depend not only on its sensitivity and specificity, but also the prevalence of the disease (or the pre-test probability of disease). This interactive display illustrates that inter-dependence.
Move the sensitivity, specificity, and disease prevalence sliders, and watch the positive and negative predictive value sliders change in response. The mosaic plot on the right shows the positive tests (T+, in maroon) and negative tests (T-, in orange) in a population.
Diseased individuals (D+) are on the right column and non-diseased (D-) on the left column.