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- Back Calculation Method:“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 1600 x .05 = 80 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: .85 x 1520 = 1292 . . . . Then 1520 - 1292 = 228 |