Cohort studies are based upon the existence of a common characteristic such as year of birth,graduation or marriage, within a subgroup of a population. Suppose you want to study the employmentpattern of a batch of accountants who graduated from a university in 1975, or study the fertilitybehaviour of women who were married in 1930. To study the accountants’ career paths you wouldcontact all the accountants who graduated from the university in 1975 to find out their employmenthistories. Similarly, you would investigate the fertility history of those women who married in 1930.Both of these studies could be carried out either as cross-sectional or longitudinal designs. If you adopta cross-sectional design you gather the required information in one go, but if you choose thelongitudinal design you collect the required information at different points in time over the study period.Both these designs have their strengths and weaknesses. In the case of a longitudinal design, it is notimportant for the required information to be collected from the same respondents; however, it isimportant that all the respondents belong to the cohort being studied; that is, in the above examples theymust have graduated in 1975 or married in 1930.