Introduction
A cohort study is a type of observational study design. The term “cohort” is derived from the Latin word “Cohors”- “a group of soldiers.” The term “cohort” refers to a group of people who have been included in a study by an event that is based on the definition decided by the researcher. The term was introduced by Frost in 1935 to describe a study that compared the disease experience of people born at different periods, in particular the sex and age-specific incidence of tuberculosis For example; a cohort of people born in California in the year 2010 can be called as “birth cohort.”
This type of study can be done by going ahead in time from the present that is known as prospective cohort study or, alternatively, by going back in time to comprise the cohorts and following them up to the present known as retrospective cohort study. A cohort study is the best way to identify incidence and natural history of a disease, and can be used to examine multiple outcomes after a single exposure. However, this type of study is less useful for examination of rare events or those that take a long time to develop.
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Advantages and Disadvantages of Cohort study
Advantages
- Gather data regarding sequence of events; can assess causality
- Examine multiple outcomes for a given exposure
- Good for investigating rare exposures
- Can calculate rates of disease in exposed and unexposed individuals over time (eg. incidence, relative risk )
Disadvantages
- Large numbers of subjects are required to study rare exposures
- Susceptible to selection bias
- Susceptible to lose to follow up or withdrawals.
- Prospective cohort study may be expensive, may require long duration for follow-up which can make it difficult to maintain follow-up
- Whereas, a Retrospective cohort study is susceptible to recall bias or information bias and less control over variables.
References
- Song JW, Chung KC. Observational studies: cohort and case-control studies. Plast Reconstr Surg. 2010;126(6):2234-2242. doi:10.1097/PRS.0b013e3181f44abc
- https://www.cdc.gov/globalhealth/healthprotection/fetp/training_modules/19/desc-and-analytic-studies_ppt_final_09252013.pdf
- Setia MS. Methodology Series Module 1: Cohort Studies. Indian J Dermatol. 2016;61(1):21-25. doi:10.4103/0019-5154.174011
- Grimes DA, Schulz KF. Cohort studies: marching towards outcomes. Lancet. 2002;359(9303):341-345. doi:10.1016/S0140-6736(02)07500-1
- Doll R. Cohort studies: history of the method. I. Prospective cohort studies [published correction appears in Soz Praventivmed 2002;47(2):90]. Soz Praventivmed. 2001;46(2):75-86. doi:10.1007/BF01299724