A cohort study is an observational and longitudinal study that follows a defined group of people and examines how their outcomes differ over time.
Researchers typically compare groups with different exposures or characteristics to investigate whether those factors are associated with particular outcomes.
Cohort studies are commonly used in medicine, public health, epidemiology, psychology, and social science.
How do cohort studies work?
Researchers identify a group, or cohort, and collect information about participants’ exposures, characteristics, and outcomes. In a prospective cohort study, participants are followed forward in time to observe future outcomes. In a retrospective cohort study, researchers use existing records to examine exposures and outcomes that have already occurred.
A cohort study may involve:
Grouping participants based on an exposure or characteristic
Following participants over a period of time
Tracking the development of specific outcomes
Comparing outcome rates between exposed and unexposed groups
Examining multiple outcomes related to the same exposure
Using existing medical, administrative, or other records to study past outcomes
For example, researchers might follow smokers and non-smokers to compare rates of lung disease, track workers with different levels of chemical exposure, or use medical records to examine long-term outcomes among patients who received different treatments.
Strengths of cohort studies
Cohort studies allow researchers to:
Observe how exposures and outcomes develop over time
Establish whether an exposure occurred before an outcome
Study multiple outcomes associated with the same exposure
Measure how frequently new outcomes occur within a population
Study rare or uncommon exposures
Examine exposures that would be impractical or unethical to assign experimentally
Study participants in real-world settings
Cohort studies can be especially useful for investigating potential risk factors and understanding how exposures are associated with later outcomes.
Limitations of cohort studies
Because researchers do not randomly assign participants to exposures, other differences between groups may influence the results. These confounding factors can make it difficult to determine whether an exposure directly caused an outcome.
Cohort studies may also require large numbers of participants or long follow-up periods, particularly when the outcome being studied is uncommon or takes a long time to develop.
Participants may drop out over time, which can affect the results if those who leave the study differ from those who remain.
Retrospective cohort studies also depend on the completeness and accuracy of existing records, which may limit what researchers are able to measure or account for.