True statements concerning cohort studies include all the following except: (AIIMS Nov 09)
|A||Cohort studies are longitudinal in design|
|B||Subjects are selected on the basis of characteristics present before the onset of the condition being studied|
|C||Subjects are observed over time to determine the frequency of occurrence of the condition under study|
|D||Cohort studies are necessary to estimate the prevalence of disease|
a. In cohort studies, a group of subjects is defined on the basis of certain baseline characteristics and followed over time for the development of the disease (or other outcome) under study.
b. The incidence of disease in subjects with various characteristics can then be compared with the incidence in subjects without those characteristics.
c. If a suitable cohort can be identified from past records, a retrospective cohort study is possible.
d. Cohort studies are not necessary to estimate prevalence. The prevalence of disease is the proportion of the population who have it at one point in time. Prevalence can be estimated from cross-sectional studies.