What is a Cohort Study?

By Julia Simkus, published Dec 31, 2021

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A cohort study is a type of longitudinal study in which researchers monitor and observe a chosen population over an extended period of time.

In cohort studies, the participants must share a common factor or characteristic such as age, demographic, or occupation. A “cohort” is a group of subjects who share a defining characteristic.

Cohort studies are observational, so researchers will follow the subjects without manipulating any variables or interfering with their environment.

This type of study is beneficial for medical researchers, specifically in epidemiology, as scientists can use data from cohort studies to understand potential risk factors or causes of a disease.

Before any appearance of the disease under investigation, medical professionals will identify a cohort, observe the target participants over time, and collect data at regular time intervals.

Weeks, months, or years later, depending on the duration of the study design, the researchers examine any factors that differed between the individuals who developed the condition and those who did not.

They can then determine if an association exists between an exposure and an outcome, and even identify disease progression and relative risk.

Advantages

Determine cause and effect relationships

Because researchers are studying groups of people before they develop an illness, they can discover potential cause and effect relationships between certain behaviors and development of a disease.

Provide extensive data

Cohort studies enable researchers to study causes of disease and identify multiple risk factors associated with a single exposure. These studies also can reveal links among different types of diseases and risk factors.

Enable studies of rare exposures

Cohort studies can be very useful for evaluating the effects and risks of rare diseases or unusual exposures, such as toxic chemicals or adverse effects of drugs.

Can measure a continuously changing relationship between exposure and outcome

Because cohort studies are longitudinal, researchers can study changes in levels of exposure over time and any changes in outcome, providing a deeper understanding of the dynamic relationship between exposure and outcome.

Limitations

Time consuming and expensive

Cohort studies usually require multiple months or years before researchers are able to identify causes of a disease or discover significant results. Because of this, they are often more expensive than other types of studies. Retrospective studies, though, tend to be cheaper and quicker than prospective studies as the data already exists.

Require large sample sizes

Cohort studies require large sample sizes in order for any relationships or patterns to be meaningful. Researchers are unable to generate results if there is not enough data.

Prone to bias

Because of the longitudinal nature of these studies, it is common for participants to drop out and not complete the study. The loss of follow-up in cohort studies means researchers are more likely to incorrectly estimate the effects of an exposure on an outcome.

Unable to discover why or how a certain factor is associated with a disease

Cohort studies are used to study cause and effect relationships between a disease and an outcome. However, they do not explain why the factors that affect these relationships exist. Experimental studies are required to determine why a certain factor is associated with a particular outcome.  

Examples

• The Framingham Heart Study Studied the effects of diet, exercise, and medications on the development of hypertensive or arteriosclerotic cardiovascular disease, in a longitudinal population-based cohort.
• The Whitehall Study The initial prospective cohort study examined the association between employment grades and mortality rates of 17139 male civil servants over a period of ten years, beginning in 1967. At the time the Whitehall Study was conducted, there was no requirement to obtain ethical approval for scientific studies of this kind.
• The Nurses’ Health Study - Researched long-term effects of nurses' nutrition, hormones, environment, and work-life on health and disease development.
• The British Doctors Study - This was a prospective cohort study which ran from 1951 to 2001, investigating the association between smoking and the incidence of lung cancer.
• The Black Women's Health Study - Gathered information about the causes of health problems that affect Black women.
• Millennium Cohort Study - Found evidence to show how various circumstances in the first stages of life can influence later health and development. The study began with an original sample of 18,818 cohort members.
• The Danish Cohort Study of Psoriasis and Depression - Studied the association between psoriasis and onset of depression.
• The 1970 British Cohort Study - Followed the lives of around 17,000 people born in England, Scotland and Wales in a single week of 1970.

Frequently asked questions about cohort studies

Sources

Barrett D, Noble H. What are cohort studies? Evidence-Based Nursing 2019; 22:95-96.

Kandola, A.A., Osborn, D.P.J., Stubbs, B. et al. Individual and combined associations between cardiorespiratory fitness and grip strength with common mental disorders: a prospective cohort study in the UK Biobank. BMC Med 18, 303 (2020). https://doi.org/10.1186/s12916-020-01782-9

Marmot, M. G., Rose, G., Shipley, M., & Hamilton, P. J. (1978). Employment grade and coronary heart disease in British civil servants. Journal of Epidemiology & Community Health, 32(4), 244-249.

Rosenberg, L., Adams-Campbell, L., & Palmer, J. R. (1995). The Black Women's Health Study: a follow-up study for causes and preventions of illness. Journal of the American Medical Women's Association (1972), 50(2), 56-58.

Samer Hammoudeh, Wessam Gadelhaq and Ibrahim Janahi (November 5th 2018). Prospective Cohort Studies in Medical Research, Cohort Studies in Health Sciences, R. Mauricio Barría, IntechOpen, DOI: 10.5772/intechopen.76514. Available from: https://www.intechopen.com/chapters/60939

Setia M. S. (2016). Methodology Series Module 1: Cohort Studies. Indian journal of dermatology, 61(1), 21–25. https://doi.org/10.4103/0019-5154.174011

Zabor, E. C., Kaizer, A. M., & Hobbs, B. P. (2020). Randomized Controlled Trials. Chest, 158(1). https://doi.org/10.1016/j.chest.2020.03.013