Last update:
20-Apr-2012

Epidemiological studies include experimental and non experimental studies. Experimental studies include clinical trials, field trials and community intervention trials, while non experimental studies include quasi experimental studies, follow-up studies (in which subjects are selected with reference to their determinant status), "case-control" studies (in which subjects are selected in reference to their illness status), cross-sectional studies and ecological studies. The "case-control" study aims at achieving the same goals as a follow-up study but more efficiently, using "sampling". Properly carried out, "case-control" studies provide information that mirrors what could be learned from a follow-up study, usually at considerably less cost and time. In "case-control" studies, the source population is the population that gives rise to the cases included in the study. The control series should be sampled from the person-time of the source population so that the determinant distribution of the controls sampled mirrors the determinant distribution of the person-time in the source population. There are a number of options in "case-control" studies for selecting a control series: (a) Population controls, (b) random-digit dialing, (c) neighborhood controls, (d) friends controls, (e) hospital- or clinic-based controls and (f ) dead people, among others. In a "case-control" study, one can calculate the quasi incidence-densities ratio which is a valid estimation of the real incidence-densities ratio in a population without having to obtain individual information on every person in the source population.

Key words: "Case-control" studies, Incidence-densities ratio, Matching, Source population, Study population.