A cohort study is an observational longitudinal research design in which a defined group of participants (a cohort) is followed over time to understand how certain exposures or characteristics influence the development of outcomes. There is no intervention and no randomisation; the researcher simply observes what occurs naturally within the population.

Cohort studies are especially useful when the aim is to understand whether an exposure (such as a behaviour, environmental factor, or occupational role) is associated with later outcomes. Because exposure is measured before the outcome occurs, cohort studies are considered the gold standard among observational designs.

Types of cohort studies

Cohort studies can be designed in two main ways: prospectively or retrospectively.

Prospective cohort studies

In a prospective cohort study, participants are enrolled before the outcome has occurred. The researcher identifies who is exposed and who is not, ensures that everyone is outcome-free at baseline, and then follows participants into the future to observe who develops the outcome.

This approach allows the researcher to control data collection methods and to monitor outcomes as they emerge.

Retrospective cohort studies

In a retrospective cohort study, both the exposure and the outcome have already occurred by the time the study is designed. Researchers use existing records (such as medical files or employment records) to determine past exposure status and assess whether individuals developed the outcome.

Retrospective cohort studies are typically faster, more practical, and more cost-effective because the follow-up period has already taken place.

Key features of cohort studies

• Observational: No intervention or manipulation
• Longitudinal: Follows participants over time
• Temporal sequence: Exposure occurs before outcome
• Population-based: Uses defined exposed and unexposed groups
• Flexible: Allows multiple outcomes to be studied from the same exposure

Benefits of cohort studies

• Longitudinal nature provides strong evidence for temporal relationships
• More cost-effective than randomised trials, especially retrospective cohorts
• Ethically safe because exposures are not assigned
• Allows study of multiple outcomes from a single exposure
• Suitable for studying rare exposures when the sampled cohort is well defined
• Useful when randomisation is not possible or ethical
• Prospective cohorts provide researcher control over data collection quality
• Retrospective cohorts serve as useful pilots for future prospective studies

Limitations of cohort studies

• Selection bias may be present
• Confounders may influence the relationship between exposure and outcome
• Prospective cohort studies can be time-consuming and costly
• Long follow-up periods may be needed, especially for rare outcomes
• Loss to follow-up is a common challenge
• Retrospective studies may rely on incomplete or low-quality records
• No randomisation is performed
• Blinding is often difficult
• Cannot determine cause and effect, although cohort studies can support causal hypotheses

When should you use a cohort study?

Use a cohort study when you want to:

• Understand how exposures influence outcomes over time
• Evaluate risk factors for diseases or conditions
• Study multiple outcomes that may arise from a single exposure
• Investigate rare exposures that are already naturally occurring
• Explore potential causal pathways when randomisation is not appropriate