Distributions of Test Results

Patients with disease are shown in the upper distribution; patients without disease are shown in the lower distribution. For patients with disease, the region beneath the distribution of results that lies to the right of (above) the cutoff criterion corresponds to the test’s true-positive rate (ie, its sensitivity); the region that lies to the left of (below) the criterion corresponds to the false-negative rate. For patients without disease, the region to the right of the cut-off criterion corresponds to the false-positive rate, and the region to the left corresponds to the true-negative rate (ie, its specificity). For 2 overlapping distributions (eg, patients with and without disease), moving the cutoff criterion line affects sensitivity and specificity, but in opposite directions; changing the cutoff criterion from 1 to 2 decreases the number of false negatives (increases sensitivity) but also increases the number of false positives (decreases specificity).