Statistical analysis of case-control studies of communicable diseases.

At the Communicable Disease Surveillance Centre most outbreak investigations are carried out by questioning cases and unaffected 'controls' to look for associations with possible sources of infection. Illness rates are compared in those exposed with those not exposed to a possible risk factor, using statistical techniques appropriate to the survey design. Significance testing to obtain evidence of the source of transmission is made as quickly as possible so that action may be taken. Microbiological corroboration is sought wherever possible. Unlike chronic disease epidemiology the estimation of odds ratios and relative risk is seldom of primary importance. Examples are given of the analysis of three types of study. Firstly where the whole population is interviewed and then where cases are matched 1:1 and 1:M with controls, including an example with missing data, ie variable numbers of controls.