Cohort study

A cohort study is a type of analysis in which one group with something in common is studied in comparison with another similar group that does not have that same trait or characteristic. For example, a study may compare how many children with pets develop allergies compared to children who do not have pets. This type of study allows researchers to study several factors at one time. However, cohort studies are expensive and time-consuming to conduct and are prone to being affected by people who change their behaviors and no longer fit the group criteria.

Background

The type of study that is known as a cohort study has been used for many centuries. However, the study acquired the name cohort in 1935 after American physician and epidemiologist Wade Hampton Frost used the term to refer to studies of patients in different populations who developed tuberculosis. Frost was interested in determining the incidence of tuberculosis in people of specific ages and genders; these criteria defined the cohorts, or populations, that he wanted to study.

The word cohort comes from the medieval French word cohorte, which came from the Latin cohortem, meaning "an enclosed yard or courtyard." The term was expanded during ancient times to refer to specific units of Roman infantry soldiers who lived and fought together in an enclosed group. In the early eighteenth century, the word came into use in American English as a group of people who had some characteristic in common. It was in this sense of the word that Frost chose to apply it to his studies because his research relied on the analysis of a group of people who shared a common trait.

Overview

A cohort study is a type of longitudinal study. Longitudinal studies follow the subjects from exposure of what is being studied to an outcome. In other words, they analyze how things develop over time, not how they are in one particular moment in time.

Cohort studies specifically study one group of people that has been exposed to something and another group that has not been exposed. For this reason, they are very commonly used to analyze health conditions. Researchers might study how often and how quickly a disease develops in people who have been exposed to some potential risk factor versus those who have not. For instance, researchers might want to study one group of people who eat meat and another that does not to see how often members of each group develop cancer, diabetes, heart conditions, or any other disease.

There are two main types of cohort studies: prospective studies and retrospective studies. In a prospective study, the participants are chosen and then followed over a period of time to see how their conditions change with exposure to whatever is being studied. In a retrospective study, the participants are studied and compared after they have been exposed to the object of the study. To determine the effects of eating meat versus not eating meat, for instance, a prospective study would start with a group of people who have chosen to eat meat and another who have chosen not to eat meat and take records over several months or years to see how their conditions change. A retrospective study would choose people who have already been eating meat or abstaining from meat and study their medical records over a past period of time to determine any changes that might have resulted from the diet.

To begin a cohort study, the researchers first determine what they want to study and what populations they want to compare. People are then chosen to meet those criteria. The two groups must be as identical as possible except for the specific factor being studied; therefore, each group will contain people of similar ages, genders, ethnicity, and other like characteristics. Those in each group will also have similar exposure or lack of exposure to the factor being studied. For example, in the group of meat eaters, all would eat a variety of meats and none would limit their diet to one type, such as poultry, and the non-meat eaters would be vegetarians or vegans, but not both.

Meeting these characteristics poses one of the most significant challenges of cohort studies. It can be difficult to assess the level of exposure each participant has had to the factor being studied. For instance, some people in the diet study may eat one serving of meat a day when others have multiple servings. People may also change their habits during the course of the study, skewing the results.

Measuring the outcome can also be difficult, depending on the type of study. For example, people who develop a health condition may be too overwhelmed with the symptoms to continue in the study, leading them to stop participating without notifying the researchers of the reason. This will alter the accuracy of the results. Another factor that could influence the outcome is called the Hawthorne effect or observer bias. This occurs when people who know they are being studied alter their other behaviors. For example, even those in the meat-eating group might start eating more fruits and vegetables, affecting their overall health.

The outcome of a cohort study can also be affected by unintentional bias in choosing the subjects. For instance, if all the participants in the meat versus no meat study are chosen from people who work for certain companies or attend college, the results might not be truly representative of the general population. People who are not well enough to work or go to college are excluded, which is a form of bias, even if it is unintentional.

Other limitations of the cohort study are that they require time and are therefore expensive to conduct. They do not work well for studying rare conditions or situations because it is difficult to find large enough cohort groups. However, cohort studies also have a number of advantages. They may be the only way to study the effects of conditions such as drug or alcohol abuse or how a disease progresses in the presence of certain variables because it would be unethical to create these conditions in people just to study them. People can also be studied for multiple variables at one time. For example, one group of people born in a certain year can be analyzed to see how often they develop specific diseases.

Bibliography

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