The most important characteristics that you should look for to identify a cohort are the following:
- It is an observational study (the investigator is an observer and does not intervene)
- It follows participants over time (several months, or even years)
- It compares the incidence of the outcome (i.e. the number of participants who developed that outcome over the follow-up period) between exposed and unexposed groups
The objective of a cohort study is to estimate if being exposed to a certain risk factor (or treatment) influences the risk of developing the outcome.
A cohort study follows participants belonging to:
- 2 groups: This is the classic case where a cohort follows a group of participants exposed to a certain risk factor and another group of unexposed participants. A cohort can also follow 2 groups regardless to their exposure status (for example males and females)
- More than 2 groups: The cohort can also follow several groups, each representing:
- a different level of exposure to a risk factor
- or an age category, etc.
- A group of people with the same occupation, job, or any subgroup of interest: In this case the cohort starts with a group of participants which their exposure status will be determined later
- A sample from the general population, chosen at random or not (convenience sample or made of people willing to participate)
No matter how these participants were chosen, the sample should always represent the target population. Otherwise the results won’t generalize well and the study would be a waste of time and resources.
Note that the number of participants in a cohort can be either fixed or dynamic:
- Fixed: the number cannot change once the participants are recruited at the start of the study
- Dynamic: participants can be added to the study within the follow-up period
The outcome of interest can be:
- A disease
- An event (like initiating smoking or stopping smoking)
- A score (or a change in the score) on a particular test (IQ test, blood pressure, etc.)
The follow-up period
Sometimes it is tricky to identify if the study follows participants over time or not.
To illustrate this point, I’ll give you an example.
Suppose you read the following sentence in the methods section of a study:
“Participants are surveyed over a period of two weeks to record their emotional status and caloric intake”.
The question is:
Are these participants “followed” over a period of 2 weeks? Is this a cohort study?
In fact, anytime we have to collect data for any kind of study (cross-sectional or cohort) it cannot be instantaneous, it has to happen over some period of time. In a cross-sectional study, we can collect data over a period of many days and average them out in order to reduce the noise in our data (an example would be blood pressure measurements). Still, this is not a cohort study!
A cohort will follow participants over much longer periods of time, like months or years and the measurement of exposure will precede the measurement of the outcome (unlike a cross-sectional study where exposure and outcome are both measured at the same time).
Characteristics of different types of cohort studies
We have 3 different types of cohorts and they differ by whether or not they use historical data and how they use it:
- Prospective cohort
- Retrospective cohort
- Mixed cohort
1. Prospective cohort
The prospective cohort (or simply cohort) begins by identifying the exposure status of participants. It then follows them over time and the outcome will occur in the future with respect to the starting date of the study. (see figure below)
At the start of the study, the outcome has not occurred yet.
Gives the most freedom in designing and planning a cohort.
Takes a long time (as we have to wait for the outcome to occur), and is more expensive relative to the other types of cohort designs.
2. Retrospective cohort
The retrospective cohort uses historical data about individuals who were followed in the past before the study even started (perhaps they participated in a past study). At the start of the retrospective cohort, the investigators will not follow participants over time, instead they will be determining which participants developed the outcome.
Otherwise, same as with a prospective cohort, we will be comparing the incidence of the outcome between the exposed and unexposed groups.
The follow-up period occurred before the study started.
The objective of a retrospective cohort is to shorten the follow-up time and get the results sooner and therefore cheaper than with a prospective cohort.
The study duration will be as long as it takes to determine who has the outcome and who has not.
Relies on past measurements that may be subject to bias or may not be optimal for our specific study.
This design may be subject to temporal bias — difficulty in determining whether the exposure occurred before the outcome or not, which may invalidate any conclusion about causality.
3. Mixed cohort
A mixed cohort is a combination of both prospective and retrospective designs. The investigator uses historical data to determine the exposure, AND then follows the participants over time before determining who developed the outcome of interest and who didn’t.
The exposure occurred in the past and is determined using historical data, and the outcome will occur in the future (with respect to the start date of the study) after a period of follow-up.
The follow-up period is shorter than that of a prospective cohort, so we get the results sooner and cheaper compared to a prospective design.
And because the outcome will be measured in the future, we will not be relying on past measurements of the outcome.
By using past exposure data, we will not have the freedom of design as with a prospective cohort.
And because we still need to follow participants over time, the duration of the study will be longer compared to a retrospective design.
Note: How to distinguish between prospective and retrospective cohorts?
In a prospective cohort, we can still ask participants for information about things that happened in the past. This does not make the design retrospective or mixed. The biggest difference between prospective and retrospective designs is that in a prospective design, the follow-up period occurs after the start of the study and in a retrospective design, the follow-up period occurred before the study even started.
- David Celentano, Moyses Szklo. Gordis Epidemiology. 6th edition. Elsevier; 2018.
- Szklo M, Nieto FJ. Epidemiology: Beyond the Basics. 4th edition. Jones & Bartlett Learning; 2018.
- Hackshaw A. A Concise Guide to Observational Studies in Healthcare. 1st Edition. Wiley-Blackwell; 2015.