In the context of diagnostic testing, concepts such as sensitivity, specificity, predictive values, likelihood ratios and more are all interconnected, but precisely how can be confusing to the nonstatistician. This paper presents a graphical explanation. Bayes' rule, or theorem, ties several of these concepts together and should have a role in careful diagnostic thinking. It too can be understood in terms of the "two-by-two diagram" presented here.
Keywords: Bayes’ theorem; diagnostic tests; diagrams; likelihood ratio; odds; posttest probability; predictive value; pretest probability; sensitivity; specificity.