One way of conceptualizing the difference between cohort and case-control studies

I always struggled with biostatistics, but I’m thankful for Rahul Patwari and his excellent video on Odds Ratios and Risk Ratios that visualizes the conceptual differences here. I think that one reason I had trouble remembering the difference between the design of cohort and case-control studies was that the terminology seemed inconsistent to me. However, seeing these concepts explained visually turned on a lightbulb for me again.

Per Merriam-Webster, one definition of a cohort is “a group of warriors or soldiers” (or students, if you are in graduate school). In other words, a cohort is a group of people who gather to perform a mission or task and go forward to accomplish that task. A cohort in a cohort study is a group that is intentionally exposed to an intervention (or not, in the case of the control group) and tracked through time in order to determine the outcome(s) of that exposure. In my mind, I refer to cohort studies as “cohort-control studies”.

On the other hand, a case-control study takes people with who experienced or are experiencing an outcome (the case group) and people who did not experience that outcome (the control group) and looks backward in time to whether those people had a specific exposure in the past prior to said outcome.

Comparing these two study designs in this way makes me wonder if case-control studies were not called “case studies” so as to be consistent with “cohort studies” simply due to the fact that the term “case study” may have already referred to what Merriam-Webster explains as “an intensive analysis of an individual unit (such as a person or community) stressing developmental factors in relation to environment”. Food for thought.

Hope that helps! Keep working hard.

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