A cohort represents a group of people followed over time. In cohort studies, medical, health, behavioural and contextual information is collected systematically. Data may be obtained e.g. from population surveys, clinical studies or medical treatments and interventions. Biological material may be stored in Biobanks.
The coverage of data is usually defined to answer specific research questions. Studies are conducted by different teams of researchers of different institutions. Accordingly, the definition of variables, measures, classifications, the comprehensiveness and quality of data differs between different cohorts.
The relevance and statistical reliability of findings depend both on the size of the cohorts and the quality of the data. Often results are not transferable to other populations or regions. Harmonising data over several cohorts, however, can greatly extend the significance of the results.
In particular, research on rare diseases, or research supporting personalised medicine, requires large cohorts in order to find a statistically representative number of individuals who share rare characteristics.