Fagan's Nomogram

Fagan's nomogram (Fagan, 1975) is a graphical tool for applying Bayes' theorem to diagnostic testing. It converts a pre-test probability — your estimate of disease probability before the test — into a post-test probability using the test's likelihood ratio (LR). A positive LR above 1 raises the probability; a negative LR below 1 lowers it. Draw a straight line from the pre-test probability through the likelihood ratio and read the post-test probability where it crosses the right-hand axis.

Use the interactive nomogram below: drag the marker on the Pre-Test Probability or LR axis — or type values in the fields on the right — and the post-test probability updates automatically.