The cohort study is an observational epidemio logical study which selects the specific study pop ulation attempting to study the relationship between an exposure to the purported cause and the subsequent risk of developing disease in accor dance of time direction. Since Doll and 11111 had studied the association between lung cancer and smoking behavior among the British medical doc tors in the years of the last fifties, the defined group cohort studies analysing the relationshipbetween an exposure and the occurence of a disease have become very popular. Cohort studies can be classified as either prospective or retrospective, depending on the temporal relationship between the initiation of the study and the occurence of the disease.
Cohort studies are admitted as the very valuable studies for demonstrating the association between an exposure and a disease because it is possible to drive relative and attributable risks and often incidence measures.
They can even examine multiple effects of a single exposure. However, they are usually expensive to carry out and large cohorts are required for rare diseases in addition to the time consuming works. There are also very significant prob lems associated with selection of appropriate groups to be studied as far as complete ascertainment of disease occurence in them. Usually it is necessary that we must compromise to provide the opportunity for various types of bias such as selection bias, follow-up bias, information bias or misclassification, confounding bias and post hoc bias to occur which can result in incorrect conclu sions. Only the success of a cohort study would be expected when the investigator pay the deep care in recognizing and correcting for these biases.
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