Cohort analysis refers to a social scientific technique which studies a population that shares common characteristics, over time. 
 Cohort analysis usually begins at birth and traces the development of cohort members until they reach a certain age. 
Other /More definition:
Cohort analysis refers to a  social scientific technique which studies a population that shares common characteristics, over time. Cohort analysis usually begins at birth and traces the development of cohort members until they reach a certain age.