Cohort study

A group of people who share a common characteristic is called a cohort.

For example, people born in the year 1981 make up a birth cohort, and people who work in construction make up an occupational cohort.

Now, cohort studies or longitudinal studies are a type of study design that follows a cohort of people over time to figure out if there’s an association between an exposure and an outcome.

Typically, cohort studies look at individuals in a cohort who have a certain exposure, as well as individuals in a cohort who have not had that exposure, to compare their rates of a certain outcome in the future.

For example, let’s say we want to figure out if there’s a relationship between smoking cigarettes and developing lung cancer.

To do this, we could follow 100,000 individuals that smoke cigarettes, the exposed group, and 100,000 individuals that don’t smoke cigarettes, the non-exposed group, for ten years.

After ten years, let’s say that 82 of the 100,000 people - 0.082% - who smoked developed lung cancer, and only 3 of the 100,000 people - 0.003% - who didn’t smoke developed lung cancer.

We can then compare the groups by dividing the probability of lung cancer for people who smoked - 0.00082 - by the probability of lung cancer for people who didn’t smoke - 0.00003 - and determine that people that smoked had 27 times the risk of developing lung cancer during that ten period.

As it turns out, smoking is the number one risk factor of most types of lung cancer, and people who smoke are 15 to 30 times more likely to develop lung cancer than people who don’t smoke.

Now, there are two main types of cohort studies.

The first type is called prospective cohort or concurrent cohort, because individuals are followed forward in time.

An example would be if in 2018 a group of smokers and a group of non-smokers are recruited for the study.

Then the two groups are followed for ten years, until 2028, and the number of people who develop lung cancer is compared between the two groups.

Prospective cohort studies are the most common type of cohort study, and are what people usually think of when they hear about a “cohort study”.

The other type of cohort study is a retrospective cohort study, also called historical or non-concurrent cohort study.

In retrospective cohort studies, participants are recruited in the past and then followed until the present day.

This can be done either with time-travel or by simply by looking at medical records from the past.

For example, you might look at medical records to find a cohort of young adults in 2008 and then divide them into a group of people that smoke cigarettes, the exposed group, and a group of people that don’t smoke cigarettes, the non-exposed group.

Then we can follow the medical records of these individuals over the next ten years, until 2018, and compare the number of individuals in each group who develop lung cancer.

So even though retrospective cohort studies use data from the past, they still have the same basic structure as prospective cohort studies.

In other words, they still start with a cohort of exposed and non-exposed individuals and follow them over time to assess their outcomes.

Now, this is different from case-control studies, which start with individuals who do or don’t have the outcome, and then use data to investigate past exposures.

This can get a bit confusing because case-control studies are sometimes called retrospective studies, and cohort studies can be either prospective cohort or retrospective cohort studies.

One advantage of cohort studies is that they’re able to clearly show the timing or temporality of the relationship between the exposure and the outcome.

For example, out of 200 people, 100 that smoke and 100 that don’t smoke, none of them have lung cancer at the beginning of the study.

But after ten years, more individuals that smoke develop lung cancer compared to individuals that don’t smoke, so it’s pretty clear that smoking happened first and lung cancer happened second.

Because of this, cohort studies are usually preferred over other study designs, like cross-sectional studies, which measure the outcome and the exposure at the exact same time.