What Is a Cohort Study?

A cohort study often looks at 2 (or more) groups of people that have a different attribute (for example, some smoke and some don't) to try to understand how the specific attribute affects an outcome. The goal is to understand the relationship between one group's shared attribute (in this case, smoking) and its eventual outcome.

Cohort Study Design

There are two categories of evidence-based human medical research:

Experimental research: This involves a controlled process through which each participant in a clinical trial is exposed to some type of intervention or situation—like a drug, vaccine, or environmental exposure. Sometimes there is also a control group that is not exposed for comparison. The results come from tracking the effects of the exposure or intervention over a set period of time.

Observational research: This is when there is no intervention. The researchers simply observe the participants' exposure and outcomes over a set period of time in an attempt to identify potential factors that could affect a variety of health conditions.

Cohort studies are longitudinal, meaning that they take place over a set period of time—frequently, years—with periodic check-ins with the participants to record information like their health status and health behaviors.

They can be either:

• Prospective: Start in the present and continue into the future
• Retrospective: Start in the present, but look to the past for information on medical outcomes and events

Purpose of Cohort Studies

The purpose of cohort studies is to help advance medical knowledge and practice, such as by getting a better understanding of the risk factors that increase a person's chances of getting a particular disease.

Participants in cohort studies are grouped together based on having a shared characteristic—like being from the same geographic location, having the same occupation, or having a diagnosis of the same medical condition.

Each time the researchers check-in with participants in cohort trials, they're able to measure their health behaviors and outcomes over a set period of time. For example, a study could involve two cohorts: one that smokes and the other that doesn't. As the data is collected over time, the researchers would have a better idea of whether there appears to be a link between a behavior—in this case, smoking—and a particular outcome (like lung cancer, for example).

Strengths of Cohort Studies

Much of the medical profession's current knowledge of disease risk factors comes from cohort studies. In addition to showing disease progression, cohort studies also help researchers calculate the incidence rate, cumulative incidence, relative risk, and hazard ratio of health conditions.

• Size: Large cohort studies with many participants usually give researchers more confident conclusions than small studies.
• Timeline: Because they track the progression of diseases over time, cohort studies can also be helpful in establishing a timeline of a health condition and determining whether specific behaviors are potential contributing factors to disease.
• Multiple measures: Often, cohort studies allow researchers to observe and track multiple outcomes from the same exposure. For example, if a cohort study is following a group of people undergoing chemotherapy, researchers can study the incidence of nausea and skin rashes in the patients. In this case, there is one exposure (chemotherapy) and multiple outcomes (nausea and skin rashes).
• Accuracy: Another strength of cohort studies—specifically, prospective cohort studies—is that researchers might be able to measure the exposure variable, other variables, and the participants' health outcomes with relative accuracy.
• Consistency: Outcomes measured in a study can be done uniformly.

Retrospective cohort studies have their own benefits, namely that they can be conducted relatively quickly, easily, and cheaply than other types of research.

Weaknesses of Cohort Studies

While cohort studies are an essential part of medical research, they are not without their limitations.

These can include:

• Time: Researchers aren't simply bringing participants into the lab for one day to answer a few questions. Cohort studies can last for years—even decades—which means that the costs of running the study can really add up.
• Self-reporting: Even though retrospective cohort studies are less costly, they come with their own significant weakness in that they might rely on participants' self-reporting of past conditions, outcomes, and behaviors. Because of this, it can be more difficult to get accurate results.
• Drop-out: Given the lengthy time commitment required to be a part of a cohort study, it's not unusual for participants to drop out of this type of research. Though they have every right to do that, having too many people leave the study could potentially increase the risk of bias.
• Behavior alteration: Another weakness of cohort studies is that participants may alter their behavior in ways they wouldn't otherwise if they were not part of a study, which could alter the results of the research.
• Potential for biases: Even the most well-designed cohort studies won't achieve results as robust as those reached via randomized controlled trials. This is because by design—i.e. people put into groups based on certain shared traits—there is an inherent lack of randomization.

A Word From Verywell

Medicines, devices, and other treatments come to the market after many years of research. There's a long journey between the first tests of early formulations of a drug in a lab, and seeing commercials for it on TV with a list of side effects read impossibly quickly.

Think about the last time you had a physical. Your healthcare provider likely measured several of your vital signs and gave you a blood test, then reported back to you about the various behaviors you may need to change in order to reduce your risk of developing certain diseases. Those risk factors aren't just guesses; many of them are the result of cohort studies.