A cohort study is a type of longitudinal study where a group of individuals (cohort), often sharing a common characteristic or experience, is followed over an extended period of time to study and track outcomes, typically related to specific exposures or interventions.
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 is investigated, medical professionals will identify a cohort, observe the target participants over time, and collect data at regular intervals.
Weeks, months, or years later, depending on the duration of the study design, the researchers will 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.
Types
Retrospective
- A retrospective cohort study is a type of observational research that uses existing past data to identify two groups of individuals—those with the risk factor or exposure (cohort) and without—and follows their outcomes backward in time to determine the relationship.
- In a retrospective study, the subjects have already experienced the outcome of interest or developed the disease before starting the study.
- The researchers then look back in time to identify a cohort of subjects before developing the disease and use existing data, such as medical records, to discover any patterns.
Prospective
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A prospective cohort study is a type of longitudinal research where a group of individuals sharing a common characteristic (cohort) is followed over time to observe and measure outcomes, often to investigate the effect of suspected risk factors.
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In a prospective study, the investigators will design the study, recruit subjects, and collect baseline data on all subjects before they have developed the outcomes of interest.
- The subjects are followed and observed over a period of time to gather information and record the development of outcomes.
Advantages
Determine cause-and-effect relationships
Because researchers study groups of people before they develop an illness, they can discover potential cause-and-effect relationships between certain behaviors and the development of a disease.
Provide extensive data
Cohort studies enable researchers to study the causes of disease and identify multiple risk factors associated with a single exposure. These studies can also reveal links between 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 the 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 estimate the effects of an exposure on an outcome incorrectly.
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. When 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 that 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 the 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
1. Are case-control studies and cohort studies the same?
While both studies are commonly used among medical professionals to study disease, they differ.
Case-control studies are performed on individuals who already have a disease (cases) and compare them with individuals who share similar characteristics but do not have the disease (controls).
In cohort studies, on the other hand, researchers identify a group before any of the subjects have developed the disease. Then after an extended period, they examine any factors that differed between the individuals who developed the condition and those who did not.
2. What is the difference between a cross-sectional study and a cohort study?
Like case-control and cohort studies, cross-sectional studies are also used in epidemiology to identify exposures and outcomes and compare the rates of diseases and symptoms of an exposed group with an unexposed group.
However, cross-sectional studies analyze information about a population at a specific point in time, while cohort studies are carried out over longer periods.
3. What is the difference between cohort and longitudinal studies?
A cohort study is a specific type of longitudinal study. Another type of longitudinal study is called a panel study which involves sampling a cross-section of individuals at specific intervals for an extended period.
Panel studies are a type of prospective study, while cohort studies can be either prospective or retrospective.
References
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
Further Information
- Cohort Effect? Definition and Examples
- Barrett, D., & Noble, H. (2019). What are cohort studies?. Evidence-based nursing, 22(4), 95-96.
- The Whitehall Studies
- Euser, A. M., Zoccali, C., Jager, K. J., & Dekker, F. W. (2009). Cohort studies: prospective versus retrospective. Nephron Clinical Practice, 113(3), c214-c217.
