A cohort is an identified group of people to be studied.
A cohort study begins by classifying exposure status among a group at risk for having the outcome. The selection of study participants is based on exposure status.
For example a large number of nurses (the cohort) who take a diet either high fiber or low in fiber are followed over time for an outcome such as colon cancer.
To to determine if fiber is a risk factor (or a protective factor) for colon cancer, the cohort is divided into those with high fiber intake and those who had low fiber intake
The relative risk of colon cancer in those with high or low fiber intakes can be calculated from such a cohort study. }
Schematic diagram of a cohort study of the relationship between low social integration (unexposed) and risk of suicide. The shaded areas represent the unexposed persons (those with low levels of social integration).
Events can be ascertained either prospectively or retrospectively.
In a prospective cohort study, both the exposed group and the unexposed (or comparison) group are followed over time as events occur.
In a retrospective cohort study, longitudinal data will have already been collected for the study group, meaning the events will have already occurred and simply must be tallied.
Statistical analysis (relative risk)for a cohort study determines whether or not the outcome rates differ between the exposed group and the comparison group. [show an example]
Example of Prospective Cohort Study
The Multi-Ethnic Study of Atherosclerosis (MESA) is one such study. Nearly 7,000 adults from 6 communities were enrolled between 2000 and 2002. Periodic follow-up examinations have been performed since the inception of the cohort. One example publication based on MESA data is by Kestenbaum et al. (5). They examined whether differences in kidney function were associated with incident hypertension in patients who lacked hypertension at baseline. Their exposure of interest, kidney function, was measured using cystatin C levels during the first data collection period. The development of hypertension was determined in subsequent examinations, ensuring that the timings of exposure and outcome were appropriate. The average follow-up in the cohort was just over 3 years. They found that higher levels of cystatin C were significantly associated with an increased risk of incident hypertension. Because this was a prospective cohort, the investigators had to wait for follow-up examinations before they could determine whether any incident disease had developed.
It involves an extended period of time defined by the point when the study begins and the point when it ends; some process occurs, and a certain amount of time is required to assess it. For this reason, both cohort and case–control studies are sometimes also called longitudinal studies.
[Cohort and case–control studies generally involve an extended period of time defined by the point when the study begins and the point when it ends; some process occurs, and a certain amount of time is required to assess it. For this reason, both cohort and case–control studies are sometimes also called longitudinal studies.
The major difference between them is the direction of the inquiry or the focus of the research question: Cohort studies are forward-looking, from a risk factor to an outcome, whereas case–control studies are backward-looking, from an outcome to risk factors.
The cross-sectional study analyzes data collected on a group of subjects at one time.
Cohort studies involve a period of time defined by the point when the study begins and the point when it ends. Cohort studies start from the exposure and look forward for the outcome.
There are two types of cohort studies.
I. Prospective cohort studies
II. Historical cohort studies
Prospective cohort studies
Schematic diagram of a cohort study. Shaded areas represent exposed persons, and unshaded areas represent unexposed persons.
References
1. Concept: Person Years - Calculating in a Cohort Study