In Medicine, what is a Cohort?

In medicine, a cohort is group of people sharing some common element and participating in a cohort research study. The group of people may be identified as possessing a common feature. This is considered different from studies where researchers collect a group of individuals that are diverse.

For example, a group may be studied that doesn’t have exposure to a common element to determine if there isn’t causality or correlation between two things, like exposure to a chemical and presence of cancer. Studies of cohorts can be retrospective and examine past behavior or they can be prospective and identify a group that is then studied into the future. Two groups might be studied at the same time, where one receives exposure or treatment and another doesn’t to determine if differences exist.

In designing experiments, researchers determine what commonalities they want groups to share. This could differ widely. For example it’s possible to make a cohort study of a single gender, age group, or create one where people bear one feature such as cigarette smoking, obesity, or daily aspirin use. If researchers are determining whether female cigarette smokers might be at greater risk for lung cancer if they took a certain antibiotic, they might make up two cohort group, and administer the antibiotic to one of them, noting results in the future

Alternately, with good research tools, scientists could perform a retrospective study by examining hospital medical records, creating cohort groups, and then determining if over time the smoking women who received the antibiotic had succumbed to lung cancer, more than those who hadn’t. This may be harder to do, especially in a study over time because records on how and where people died might not be available from the same hospital.

Another way to conduct a cohort study is to look at whether a risk factor results in a known outcome. Two groups bearing similarity in at least one way might be studied to determine if there exists appreciable differences in outcome depending on the risk factor’s presence. A group of 30-year-old males who drink regularly might be contrasted with 30-year-old males who don’t to determine if an outcome like liver damage occurs. As these groups continue to be studied, research could reveal a lot about differences or disprove suggestions that drinkers are more likely to have liver problems.

One thing noted about studies that take place over lengthy periods of time is that it can be difficult to remain in touch with study participants. Clearly explaining to cohort members that they may need to come back for additional exams or answer questions on the phone, and they should stay in touch if they plan to move, is valuable. Usually a large enough study can lose a few people, but a small cohort means each person studied is valuable and research may only be successful if each study member can continue to be reached over the length of the study.