The Importance of Prior Probabilities in Coronavirus Testing

Diagnostic tests are looked to as an essential tool in the fight to contain the coronavirus and guide the reopening of economies. Much is said about problems with gaining access to these tests. Very little is said about their accuracy. These tests are assumed to be highly accurate. However, unless a test is perfect, accuracy can be greatly affected by the prevalence of infection in the population being tested, also known as the base-rate. Other contextual factors such as symptomatology and prior exposure also need to be taken into account along with base rates. One rarely hears anything about the importance of base rates because, unless one is schooled in their importance, they are not understood, not deemed relevant, and are thus neglected. Relying on diagnostic tests without verifying their accuracy and helping people interpret them will compound the crisis already underway and delay recovery.