Define the concepts of:
Explain how the prevalence of a disease influences the PPV and the NPV of a
diagnostic test (20% of marks).
The question required a precise definition of each term. Better answers included a 2
x 2 table denoting true and false positives and negatives. Discussion of sensitivity,
specificity, positive predictive value (PPV) and negative predictive value (NPV)
needed to be consistent with accurately quoted formulae. The directional influence
of prevalence on PPV and NPV should have been stated clearly. Bayes’ Theorem
was noted in more comprehensive answers.
Syllabus: Evidence Based Medicine 2f
Recommended sources: Statistical methods for anaesthesia and intensive care,
Myles and Ginn.