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What is a Cohort Study?

By Julia Simkus, published Dec 31, 2021


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.

Types of Cohort Studies

Retrospective

  • In a retrospective study, the subjects have already experienced the outcome of interest, or developed the disease, before the start of the study.
  • The researchers then look back in time to identify a cohort of subjects before they had developed the disease and use existing data, such as medical records, to discover any patterns.

Prospective

  • In a prospective study, the investigators will design the study, recruit subjects, and collect baseline data on all subjects, before any of them 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 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

1. Are case-control studies and cohort studies the same?

While both types of studies are commonly used among medical professionals to study disease, they are not the same.

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, and then after an extended period of time, 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?

Similar to 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 time spans.

3. What is the difference between cohort and longitudinal study?

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.

About the Author

Julia Simkus is an undergraduate student at Princeton University, majoring in Psychology. She plans to pursue a PhD in Clinical Psychology upon graduation from Princeton in 2023. Julia has co-authored two journal articles, one titled “Substance Use Disorders and Behavioral Addictions During the COVID-19 Pandemic and COVID-19-Related Restrictions," which was published in Frontiers in Psychiatry in April 2021 and the other titled “Food Addiction: Latest Insights on the Clinical Implications," to be published in Handbook of Substance Misuse and Addictions: From Biology to Public Health in early 2022.

How to reference this article:

Simkus, J. (2021, Dec 31). What is a Cohort Study? Simply Psychology. www.simplypsychology.org/cohort-study.html

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