Last update:
30-Jan-2011

Epidemiological studies include both experimental and non experimental studies. Experimental studies include clinical trials, field trials and community intervention trials, while non experimental studies include quasi experimental studies, cohort studies, "case-control" studies (in which participants are selected with reference to their illness status), cross-sectional studies and ecological studies. Cohort studies are a direct analogue of the experimental studies, but in the former the investigator cannot assign the distribution of the participants on the basis of the determinant under study, and randomization is not feasible. A cohort study is one in which two or more groups of people who are free of illness at the start of the study, and who differ according to extent of exposure to a potential determinant of a certain illness are compared with respect to the incidence of that illness in each of the groups. In closed populations, incidence-proportion or incidence-density can be calculated as measures of illness occurrence, while in open populations only incidence-density can be calculated. It is important to keep both the induction time and the latent period in mind in the calculation of measures of illness occurrence. After the exposure of the population to index category of the determinant, illness does not occur until the induction time has passed. After illness occurs, there is an additional interval, the latent period, during which illness exists but has not been diagnosed.

Key words: Cohort studies, Incidence-density, Incidence-proportion, Induction time, Latent period.