As we have noted, sensitivity and specificity are properties of tests, and therefore usually do not change. Can you think of a situation when these properties might change? (Hint: if you noted a lump on exam, how sure would you be? How sure would you be if your tutor performed the same exam?)

Operator dependency: Some diagnostic tests, physical exams for example, are “operator dependent”. This means that the sensitivity and specificity will improve with more experienced operators (examiners). Here, we have assumed the same level of skill between the clinicians in the breast screening centre and the surgery clinic.

As we have also noted, predictive values change depending upon the patient population under study. As the likelihood of disease goes up, the PPV goes up. Also notice that the NPV was lower (i.e. the absence of a lump is less reassuring) in the high risk surgical clinic population.