Chapter 4 Sensitivity, Specificity, and Relatives

Poetry teaches us music, metaphor, condensation and specificity.

– Walter Mosley

4.1 Introduction

This chapter introduces several notions fundamental for disease or device testing. The sensitivity, specificity, and positive and negative predictive values of a test are measures of the performance of a diagnostic test and are intimately connected with probability calculations (estimations) and Bayes’ rule.

Although concepts such as “false positives” and “true negatives” are quite intuitive, many students and even health professionals have difficulties in assessing the associated probabilities. The following problem was posed by Casscells et al. (1978) to 60 students and staff at an elite medical school: If a test to detect a disease whose prevalence is 1/1000 has a false positive rate of 5%, what is the chance that a person found to have a positive result actually has the disease, assuming you know nothing about the person's symptoms or signs?

Assuming that the probability of a positive result given the disease is 1, the answer to this ...

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