A cohort study is a study of a group of people over time to see how outcomes differ when people are provided with different treatments and interventions. The study can also involve following groups of people who are similar except for one lifestyle factor, such as occupation or personal habits like smoking. These studies are expensive to administer and can take years to complete, but they provide very valuable observational data.
There are a number of reasons researchers may choose to use a cohort study. Unlike case studies, where people follow up after the fact by reading through documentation on medical patients, cohort studies provide an opportunity to acquire data over time, to select for specific data of interest, and to interact with the subjects in real time. Observational studies like cohort and case studies are used when there are ethical issues with experimentation, such as in cancer treatments where it is not considered reasonable to offer some people treatment while giving a control group nothing at all.
To recruit people for a cohort study, researchers identify a topic of interest and map out the study so they can approach subjects with information about what to expect. People may be compensated for their participation in the study and generally are provided with free medical care when they need to see a doctor to contribute data to the study. The length of a cohort study is a cause for concern, as having members of the cohort drop out partway through would skew the data, and people are encouraged to think carefully before consenting to study involvement.
In addition to a cohort selected on the basis of similar characteristics, such as people with similar kinds of cancer receiving the same treatment, a control group will also be assembled. This control group contains people who are more or less identical except for a key factor. For instance, people studying the use of gamma knife surgery for brain cancer could perform a cohort study on patients who received this surgery and compare those patients with people who received more conventional treatments, to see which treatment offers the best patient outcome.
These studies usually create a mass of data that can be applied in a variety of ways. Researchers involved in a cohort study may involve other researchers if they uncover interesting information in the process of the study. The results can be published over time in trade journals, as well as being summarized at the end of the study.