What’s the difference between case-control studies and cohort studies ?

I have written previously about case-control and cohort studies. This post compares them to each other.

There are some important similarities and differences, which will be useful to discuss.

Case-Control Studies

In case-control studies, you start with people who already have the disease or a particular condition (these are your cases). You add these people to a group and then you create a similar group. These would be people who do not have the condition of interest but are in every other way, similar to our cases (these are our controls).

With both groups we look back (retrospectively) at different exposures that could have caused the outcome or diease/illness of interest. We then compare these two groups and try to determine if there are any differences in the presence or magnitude of the exposure.

This is the reverse of a cohort study in that in this approach we are starting with people who already have the disease (our cases )and people who do not have the disease (our controles) and looking back in time at their exposures to determine what exposure(s) where different in both groups.

Cohort Studies

In cohort studies, you start with a rare exposure of interest. The cohort can consist of disease free individuals who you follow over time to determine the risk of a specific outcome or disease occurring. Then you find a second group of people (controls) who are in every way similar to the first group except in the exposure of interest. You have to make sure that they are the same because the goal is to compare these two groups. You follow these groups over time and you examine what outcomes emerge.

Cohort studies can either be prospective (starting before the outcome) or retrospective (starting after the exposure is known but the outcome is still not known).

Summary Table

Case-Control StudiesCohort Studies
Great for studying rare conditions or rare diseasesInefficient for studying rare diseases and diseases that take a long time to develop
The retrospective nature of case-control studies allows you to look at different exposuresIn a retrospective cohort study, the exposed group may already have the outcome of interest, but you would observe them via historical medical records to examine them at a time before the outcome occurred. The goal here is to determine if they had the exposure of interest
You do not need a large sample size. Just people who have the disease and people who do not have the disease.Can be time consuming if the researcher has to wait several years before diseases manifest in enough individuals to obtain accurate results
It doesn’t cost much to do and you can do it relatively quicklyCohort studies are expensive, both in time and money
You can compare incidence, prevalence and odds ratioCohort studies provide true incidence rates and relative risks
Not considered a strong study designConsidered to provide stronger evidence than a case control study
Prone to selection, information and recall biasLoss to follow up is a major issue for cohort studies
You can only examine one disease at a timeYou can study multiple diseases at once to determine which exposures lead to what diseases
Issues of causality and temporality. You can only suggest an association or a link.You can determine casualty because the exposure precedes the outcome or disease

I hope this post is helpful to interested readers. I started MyStudentHq as a way to learn and write about what I learn. My foray into epidemiologic study designs on this site is related to my commitment to learning and writing boldly.

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