For simplicity, however, assume a population has been
tested and assigned to the four mutually exclusive cells in
figure 1. Sensitivity, sometimes termed the detection
rate,10 is the ability of a test to find those with the disease.
All those with disease are in the left column. Hence, the
sensitivity is simply those correctly identified by the test
(a) divided by all those sick (a+c). Specificity denotes the ability of a test to identify those without the condition.
Calculation of this proportion is trickier, however. By
analogy to sensitivity, many assume (incorrectly) that the
formula here is b/(b+d). However, the numerator for
specificity is cell d (the true negatives), which is divided by
all those healthy (b+d).