“A physician wishes to assess the predictive ability of a clinical test in a population in which the prevalence of breast cancer is 5%. Test sensitivity and specificity are unchanged; that is, they equal .95 and .85, respectively”.

Step 1. Select an arbitrary sample size, for example 1600.

Step 2. Based on a prevalence P (D⁺), of 5%, calculate the number of diseased individuals:

Step 3. Calculate the number of disease-free individuals:

1600 - 80 = 1520

Step 4. Calculate the proportion of diseased individuals with a positive test, based on a sensitivity of .95:

.95 x 80 = 76 . . . . then 80 - 76 = 4

Step 5. Calculate the proportion of disease-free individuals with a negative test, based on a specificity of .85: