The proportion of correct and incorrect classifications of healthy subjects and patients is co-determined by disease prevalence in the study population. On that account the predictive value of a test in principle differs strongly in general practice, in occupational medicine and in an outpatient or clinical population. This is exemplified in the following examples, in which the sensitivity and specificity are the same, and which relate to two populations comprising 1800 subjects.
The figure below illustrates how the disease prevalence in the study population affects the above indices.