Cohort study

A cohort study is a type of observational and analytical research (prospective or retrospective) in which a comparison is made of the frequency of occurrence of an event between two groups, one of which is is exposed to a factor that is not present in the other group. The individuals that make up the study groups are selected based on the presence or absence of the assessed exposure.

In prospective cohort studies limited to the field of epidemiology, the individuals selected at the beginning do not have the disease of interest and are followed up for a certain period of time to observe the frequency of the disease in each of the groups. It is also called a monitoring study, projection study or incidence study, and its objective is to measure the association between risk factors and the disease at study. In a retrospective cohort study, the subjects are studied after the disease has occurred, using databases that have historical information on the disease and the risk factors that may have caused the disease.. Classic follow-up studies are prospective studies, characterized by the fact that the approach to the study occurs prior to the development of the disease. These are very expensive studies because they require large financial and time resources, but they are very solid, since the possibility of being biased is less than in other types of studies. Despite this, it is necessary to take great care in design aspects and avoid biases, especially that of classification and that of healthy workers.

Steps of a cohort study

1. Select a sample of healthy population studies.
2. Measure exposure variables in the sample, if the risk factor is absent or present.
3. Follow the cohort.
4. Measure the result variables, i.e. the presence or absence of disease.

Advantages of a cohort study

1. Study strange exposure factors.
2. Visualize the multiple effects that an exhibition may have.
3. To simultaneously observe the effects of several exhibitions (always and when this possibility is raised from the beginning of the study).
4. Posibilitate the sample of the time sequence between exposure and outcome.
5. Allow the estimation of incidence and relative risk.
6. Clearly establish the sequence of events of interest such as exposure- sickness.
7. Avoid the bias of survival.
8. Have better control over the selection of subjects.
9. Have more control of the measures.
10. It is characterized by movement.

Drawbacks of a cohort study

1. It often requires large sample sizes.
2. It is not efficient at rare events or with long periods of latency.
3. Your execution may require a lot of time and money.
4. It is not useful for rare diseases.
5. They are expensive studies for the number of people and the necessary follow-up time, as well as the efforts to be made to ensure that the quality of follow-up minimizes the losses among the populations that follow, keeping them at an acceptable level.

As an alternative, there is another type of cohort study: retrospective cohorts, in which the research is considered after the disease has occurred, for which both the presence of exposure and the outcome must come from historical records of high reliability in which exposure and outcome have been accurately recorded at the time of occurrence.

Examples of Cohort Studies

• The Framingham Heart Study is one of the best known cohort studies when studying cardiovascular risk factors.
• The United Kingdom Prospective Diabetes Study (UKPDS) is the cohort study on the evolution of type 2 diabetes mellitus.