An observational study known as a cohort study analyses a group of individuals over time to investigate the effects of various factors (such as exposure to a particular risk factor) on the participants’ health outcomes. Individuals in the cohort share a characteristic or lived experience, such as birth year or geographic location.
Cohort studies are observational, longitudinal studies that look into health outcomes and risk factors that are predictive.
They are not the same as clinical trials in that the participants receive no exposure, treatment, or intervention. Researchers are interested in factors that are already present in the study group they are looking into.
Participants in the study are observed over time. The disease incidence within the exposed group is compared with that within the unexposed group.
Cohort studies can only identify a correlation between a risk factor and disease, not the cause, due to their observational nature.
Prospective cohort studies and retrospective cohort studies are two of the many different cohort study types, including open, closed, and dynamic.
Cohort studies are especially valuable for identifying risk factors for illnesses. They are frequently employed in areas such as medicine and healthcare research since they can assist researchers in identifying effective methods to help prevent or treat the disease.
Cohort studies are an effective method for doing research in human populations. They are a form of longitudinal study design. Longitudinal studies are studies that track participants throughout time. People in cohort studies often share certain features, such as their geography or age.
There are several methods that researchers find participants. They might get in contact with people at random via a birth register or via postal address.
In a cohort study, participants provide information to the researchers so they can build a more comprehensive picture of the group that they are studying. Researchers ask questions to learn about the group’s demographics, or traits like age and race.
Additionally, they might collect data on the following factors:
- Medical
- Biological
- Environmental
- Social
- Genetic
- Psychological
Cohort studies are also useful for determining the relationships between environmental factors—such as chemicals in food, water, and air—and health. The World Health Organization (WHO)Trusted Source assists researchers in examining these issues through large-scale cohort studies.
Example of Cohort study
Birth Cohorts
A large-scale cohort study called the Millennium Cohort Study was started in 2000. The purpose of the study is to look into the health of children born in the UK between 2000 and 2002.
The initial cohort included around 19,000 newborns. Soon after birth, they were included in the study and had frequent follow-ups, physicals, and cognitive assessments to monitor their physical, social, and cognitive development.
There have been many conclusions drawn from the Millennium Cohort Study about a variety of social and health issues. It has specifically emphasized the significance of early childhood education for cognitive development as well as the influence of social and familial factors on the health of children.
Nurses’ Health Study
- Initiated in 1976, this long-running cohort study focused on female health, particularly investigating the effects of oral contraceptives.
- It recruited married female nurses aged 30–55 years initially, with subsequent cohorts aiming for more diversity.
- The study provided significant insights into various health aspects, including the protective effects of nuts against heart disease and the association between early adulthood weight gain and later health risks.
Framingham Heart Study
- Started in 1948, this landmark cohort study enrolled over 5,200 participants from Framingham, MA, to investigate cardiovascular risk factors.
- Additional cohorts were recruited in 1971 and 2002, contributing to the understanding of heart health.
- Ongoing research in this study focuses on exploring the impact of genetic factors on cardiovascular health risks.
When to conduct a cohort study?
Cohort studies are a form of observational study that can be both qualitative and quantitative. Depending on the research topic, they can be used for both explanatory and exploratory research.
Prospective cohort studies
Data is gathered over time in prospective cohort studies to compare the occurrence of the desired result in individuals exposed to the risk factor vs those who were not. This can assist in determining whether the risk factor and the result could be related.
Prospective cohort studies investigate the relationship between exposure and outcome by looking forward in time.
In prospective cohort studies, two groups (exposed and unexposed) are followed over time to observe new disease development.
Example of Prospective cohort studies
In a prospective cohort study, researchers compared four groups of women (two at-risk and two low-risk) to determine the likelihood of developing PTSD symptoms following a birthing event.
retrospective cohort studies
In retrospective cohort studies, participants must already have the condition or health outcome being researched before participating. After then, the study focuses on examining the health results of people who have similar exposure to the risk factor throughout time.
Retrospective cohort studies investigate the relationship between the outcome and the exposure by looking backwards in time.
In retrospective cohort studies, cohorts are defined from a previous time point, with no future follow-up. Data is collected from past records to investigate outcomes, useful for diseases with long latency periods.
Example of retrospective cohort study
In a retrospective cohort study, researchers used pre-existing data to examine the potential association between birth experiences and subsequent maternal caregiving attitudes and behaviors over a 12-month timeframe.
For your research, a cohort study might be useful if:
- A strong hypothesis exists that links an exposure to an outcome.
- You are comfortable and able to support research that takes a longer period to complete, and you have access to a huge pool of research subjects.
- The relationship between the exposure and the health result under consideration is not well understood, and/or the long-term effects have not been thoroughly examined.
- There is not too much time between exposure and outcome (raising the possibility of participant attrition and increasing study costs).
- Either the exposure you’re examining is uncommon, or you can be unable to conduct a conventional experiment design due to ethical concerns.
- The outcome isn’t that unusual.
Stages of a Study Cohort
The first step in a cohort study is to choose a group of participants, or “cohort,” from the same population who do not already have the outcome being studied but may eventually develop it.
All of the participants need to be identical, which means sharing just their exposure status.
The participants are divided into two groups: the first is the ‘exposure’ group, while the second is not exposed.
What clinical questions is this study design most suited to address?
Question Type | Study Example | |
Risks | What risk factors predict disease? | This cohort study looks into the relationship between food and lifestyle risk factors and women’s hypertension. |
Prognosis | What happens with this disease over time? | This cohort study looks at cases of psychosis recovery following a first episode. |
Aetiology | What factors cause these outcomes? | This cohort study examines prenatal exposure to environmental toxins to understand their role in predicting the occurrence of developmental disorders in children. |
Diagnosis | If the test is positive, what happens to the patient? | This cohort study looks at recently released adult who have been diagnosed with a mental illness and/or substance abuse disorder, and it explores their post-diagnosis outcomes. |
Advantages of Cohort studies
- Cohort studies directly investigate the risk of disease and its contributing factors.
- Ethically safe research design.
- Allow measurement of multiple outcomes.
- Suitable for studying rare exposures, such as exposure to chemical spills in factory.
- Effective method for establishing cause and effect relationships.
- Large sample sizes allow for confident conclusions about the link between risk factors and disease.
- Useful for identifying timelines over which behaviors contribute to disease.
- Can provide valuable insights into long-term health outcomes.
Disadvantages of Cohort studies
- Not suitable for rare diseases or those with long development times, like mesothelioma.
- Inappropriate for studying multiple exposures simultaneously.
- Complex, time-consuming, and expensive to conduct, especially when collecting prospective data over many years.
- Risk of bias due to participant dropouts and potential alterations in behavior because of study awareness.
- Analysis of data from large-scale studies can be challenging due to numerous confounding variables.
- Lack of randomization in some cohort studies reduces their robustness compared to randomized controlled trials.
- Retrospective cohort studies, while less time-consuming, are prone to sampling bias and missing data, and may have lower-quality data fields.
What are the characteristics of a strong cohort study?
- Justification for the sample size is given, and it is evident how the sample population was selected, along with the inclusion and exclusion criteria. The sample group is a true representation of the population that it is taken from.
- The study’s objective is clearly stated.
- Participants who fail to follow-up are reported and given explanations.
- The control group’s selection process, whether they were from the same sample population, and if they were matched or randomised to prevent bias and confounding are all explained in detail.
- It is clearly indicated whether or not the study was blinded, i.e. whether the investigators knew how the subject and control groups were assigned.
- There was strict adherence to methodology.
Conclusion
Cohort studies are fundamental to epidemiological research, providing insights into disease risk factors by analyzing data from thousands of participants over long periods. Disease prevention guidelines, widely adopted by healthcare professionals globally, rely on evidence from renowned studies such as the British Doctors Study, the Framingham Heart Study, and the Nurses’ Health Study. Beyond epidemiology, cohort studies are valuable in nursing research, investigating factors influencing student progression and nurses’ career development.
However, conducting cohort studies poses challenges due to their scale, complexity, and duration. Nonetheless, with careful planning and implementation, cohort studies can significantly contribute to the advancement of evidence-based healthcare.
Other articles
Please read through some of our other articles with examples and explanations if you’d like to learn more about research methodology.
Statistics
Methodology
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- Theoretical Framework
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- Triangulation
- Grounded Theory
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- Randomized Controlled Trial
- Stratified Sampling
- Ethnography
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Research
- Table of Contents
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- Synopsis
- Thesis Statement
- Research Proposal
- Research Questions
- Research Problem
- Research Gap
- Types of Research Gaps
- Variables
- Operationalization of Variables
- Literature Review
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- Questionnaire
- Abstract
- Validity
- Reliability
- Measurement of Scale
- Sampling Techniques
- Acknowledgements