What Is a Retrospective Cohort Study? | Definition & Examples

A retrospective cohort study is a type of observational study that focuses on individuals who have an exposure to a disease or risk factor in common. Retrospective cohort studies analyse the health outcomes over a period of time to form connections and assess the risk of a given outcome associated with a given exposure.

It is crucial to note that in order to be considered a retrospective cohort study, your participants must already possess the disease or health outcome being studied.

When to use a retrospective cohort study

Retrospective cohort studies are a type of observational study. They are often used in fields related to medicine to study the effect of exposures on health outcomes. While most observational studies are qualitative in nature, retrospective cohort studies are often quantitative, as they use preexisting secondary research data. They can be used to conduct both exploratoy research and explanatory research.

Retrospective cohort studies are often used as an intermediate step between a weaker preliminary study and a prospective cohort study, as the results gleaned from a retrospective cohort study strengthen assumptions behind a future prospective cohort study.

A retrospective cohort study could be a good fit for your research if:

1. A prospective cohort study is not (yet) feasible for the variables you are investigating.
2. You need to quickly examine the effect of an exposure, outbreak, or treatment on an outcome.
3. You are seeking to investigate an early-stage or potential association between your variables of interest.

Examples of retrospective cohort studies

Retrospective cohort studies are common in fields like medicine, epidemiology, and healthcare.

Advantages and disadvantages of retrospective cohort studies

Retrospective cohort studies can be a good fit for many research projects, but they have their share of advantages and disadvantages.

Advantages of retrospective cohort studies

• Retrospective cohort studies are a great choice if you have any ethical considerations or concerns about your participants that prevent you from pursuing a traditional experimental design.
• Retrospective cohort studies are quite efficient in terms of time and budget. They require fewer subjects than other research methods and use preexisting secondary research data to analyse them.
• Retrospective cohort studies are particularly useful when studying rare or unusual exposures, as well as diseases with a long latency or incubation period where prospective cohort studies cannot yet form conclusions.

Disadvantages of retrospective cohort studies

• Like many observational studies, retrospective cohort studies are at high risk for many research biases. They are particularly at risk for recall bias and observer bias due to their reliance on memory and self-reported data.
• Retrospective cohort studies are not a particularly strong standalone method, as they can never establish causality. This leads to low internal validity and external validity.
• As most patients will have had a range of healthcare professionals involved in their care over their lifetime, there is significant variability in the measurement of risk factors and outcomes. This leads to issues with reliability and credibility of data collected.

Frequently asked questions

What is the difference between a retrospective and a prospective cohort study?

The primary difference between a retrospective cohort study and a prospective cohort study is the timing of the data collection and the direction of the study.

A retrospective cohort study looks back in time. It uses preexisting secondary research data to examine the relationship between an exposure and an outcome. Data is collected after the outcome you’re studying has already occurred.

Alternatively, a prospective cohort study follows a group of individuals over time. It collects data on both the exposure and the outcome of interest as they are occurring. Data is collected before the outcome of interest has occurred.

How does recall bias affect the results of a retrospective cohort study?

Retrospective cohort studies are at high risk for research biases like recall bias. Whenever individuals are asked to recall past events or exposures, recall bias can occur. This is because individuals with a certain disease or health outcome of interest are more likely to remember and/or report past exposures differently to individuals without that outcome. This can result in an overestimation or underestimation of the true relationship between variables and affect your research.

Can a retrospective cohort study establish causality?

No, retrospective cohort studies cannot establish causality on their own.

Like other types of observational studies, retrospective cohort studies can suggest associations between an exposure and a health outcome. They cannot prove without a doubt, however, that the exposure studied causes the health outcome.

In particular, retrospective cohort studies suffer from challenges arising from the timing of data collection, research biases like recall bias, and how variables are selected. These lead to low internal validity and the inability to determine causality.