# Cohort study

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### USMLE® Step 2 style questions USMLE

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A researcher is attempting to better understand the risk factors contributing to the development of breast cancer. The researcher wants to study the effects of exposure to mercury in fish and onset of breast cancer in ten years in a population of 20,000 individuals in the Pacific Northwest. Which of the following types of studies is best suited to answer this researcher's clinical question?

### External References

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#### Cohort studies p. 258

relative risk and p. 258

### Transcript

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#### Contributors

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.

### Summary

A cohort study is a study that helps to determine a relationship between an exposure and a future outcome. In cohort studies, a group of people with a specific characteristic, such as exposure to a particular substance, are followed over time to see if they develop a specific disease or health outcome.

Cohort studies can be either prospective or retrospective. In prospective cohort studies, also known as concurrent cohort studies, individuals are followed forward in time, and the number of people who develop a particular outcome gets compared between the two groups. Next, we have retrospective cohort studies, also called historical or non-concurrent cohort studies. In retrospective cohort studies, two groups of individuals are selected in the past and followed up until the present day. Comparing two groups determines the number of individuals in each group who develop a particular outcome.

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