Overdiagnosis

In epidemiology and mass screening, overdiagnosis is the diagnosis of non-harmful disease. Overdiagnosis inflates the importance of the screening problem.

Overdiagnosis can be prevented by studying the screening program with a randomized controlled trial in which one arm of the trial the subjects are randomly assigned to the screening program and in the other arm of the trial, subjects are assigned to the control group. Examiniation of the rate of diagnosis over time detects overdiagnosis:
 * "If overdiagnosis has not occurred, the cumulative number of cases in each arm will equalize with time after screening stops (i.e., catch-up) as the counterparts of the earlier screen-detected cancers are detected symptomatically in the control arm."
 * "If overdiagnosis has occurred, the number of cases in both arms will never equalize because the excess cases in the intervention arm will have no counterparts in the control arm."