Illustration of a three source capture-recapture study. The outermost square represents the total number of infections occurring in a given population in a given time period, the second square represents the total symptomatic cases, and the innermost square represents all symptomatic cases attending healthcare. In this example, of all infected individuals attending healthcare, all cases - a will appear in at least one data source (which in this example are the laboratory database, hospital database and notifications sent to the public health agency through the notification system). a represents the number of symptomatic cases attending healthcare that were not captured by any data source and remain undiagnosed or not notified (i.e. the underreported cases). x, y, w and z cases are recorded in more than one data source with x, y and w captured in two data sources and z cases captured in 3 data sources. The true number of cases attending healthcare and that should be reported to the national level is: = cases in N + (cases in H (-w -x -z)) + (cases in L (-w -y -z)) + a. Adapted from: .