In a retrospective cohort study, the investigator looks into the exposure records of individuals to form a historic cohort population. The occurrence of outcomes in the population would then be investigated based on existing information, thus eliminating the necessity of a follow-up period, which is required for a prospective cohort study.
In a retrospective cohort study, the investigator identifies a population that has pre-existing, historical data on exposures. The investigator then looks into what happened to exposed and unexposed members of the cohort over time. The occurrence of outcomes in the exposed and unexposed groups is then compared. One major advantage of conducting a retrospective cohort study as compared to a prospective cohort study is that it takes a relatively short time to complete the study since the follow-up period has already occurred.
To estimate the association between intrauterine growth and childhood survival, Samuelsen et al (Am J Epidemiol. 1998 Nov 15;148(10):983-91) used records from Norwegian Birth Registry to obtain the birth weights of all children born between 1967-1989. The vital statistics of the children were then determined through linkage to a national death registry. The authors observed that, for children between ages 1 and 5, those with birth weights < 2.5 kg (exposed group) had 2.18 times the risk of death compared to those with birth weights ≥ 2.5 kg (unexposed). Therefore, low birth weight may be positively associated with childhood mortality.
Fundamentals of epidemiology (Univ. of North Carolina, Chapel Hill)