What Is a Cohort Study?

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In a cohort study, a group of people with a shared attribute are followed over time—anywhere from days to weeks to years—with the goal being to understand the relationship between the group's shared attribute at the beginning of the study and its eventual outcome.  

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Cohort Study Design

There are two categories of evidence-based medical research:

Experimental research: This involves a controlled process through which each person in one group of a clinical trial is exposed to some type of intervention—like a drug or vaccine—while the other is not. The results come from tracking the effects of the exposure to the intervention over a set period of time.

Observational research: This is when there is no intervention. The researcher simply observes the participants' exposure and disease status over a set period of time in an attempt to identify potential risk factors for 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 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 the same age, from the same geographic location, or having the same occupation.

Each time the researchers check 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, and 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 risk factors for various diseases 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.

Cohort studies are usually large in size, which allows researchers to draw confident conclusions regarding the link between specific risk factors and disease. And because they track the progression of diseases over time, cohort studies are also helpful in identifying and establishing a timeline of a health condition, as well as help researchers determine whether specific behaviors are potential contributing factors to disease.

In addition, 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). In situations where exposure is a rare condition, then a cohort design is a good way to study the relationship between exposure and outcomes.

Another strength of cohort studies—specifically, prospective cohort studies—is that researchers are able to measure the exposure variable, other variables, and the participants' health outcomes with relative accuracy. As a result, this keeps the measurements of the study uniform when it comes to the exposures and outcomes. 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 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 for 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 often rely on participants' self-reporting of past conditions, outcomes, and behaviors. Because of this, it is far 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 compromise 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 the potential for biases.

A Word From Verywell

Though most people use medicines, devices, and other treatments that came to the market only after years of research, many don't give much thought to the process. But there's a long journey between creating the early formulations of a drug in a lab, and when you see a commercial for it on TV, with a list of side effects read impossibly quickly. That's an example of an experimental study, but the same is true for cohort studies, which are observational.

Think about the last time you had a physical. Your doctor 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; they are the result of cohort studies.

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