Case-control Study – Observational Studies (Study Designs)

00:01 Now let's look at the case control study. The case control study sort of works backwards in time, we begin by ascertaining the outcome, who has the disease, who doesn't have the disease and we look backwards in time to see who had the exposure. In other words, who had smoking behavior in this case. So we have those individuals who are cases, the cases are those who have the disease, in this case lung cancer. The controls are those who don't have a disease, in this case people without lung cancer. We look backwards in time to see which members of both groups had a smoking behavior, how do we look backwards in time? We could ask them, were you a smoker? We can look at their medical records; those are the two commonest ways of doing so. So again, the path of the cross-sectional inquiry is to go backwards in time, chronologically we look backwards, that's the direction of inquiry. We determine the proportion who were exposed, in this case to smoking amongst our cases and the proportion who were exposed amongst the controls and we compare those two numbers. So again, here's our scenario, we have people with lung cancer and people without lung cancer. We ask how many were smokers, and how many weren't smokers.

01:18 We compare those two proportions, it's as simple as that. We get our comparative figure and we determine if that's large or small and that tells us if there is likely an association between smoking and lung cancer. Now case-control studies are useful in a variety of contexts, especially if the outcome is rare. Think about it, if the disease that I'm looking for is very rare, I can go out and find some of those, I can go to a clinic that specializes in that disease and find those cases, that can identify some other controls quite easily, people who don't have that disease and I look backwards to see what the exposure was. The problem is I can't measure incidence rates with a case-control approach, the reason is that I went out and I found those individuals with the disease, because I found them I can't say they manifested on their own. I control the numbers, so incidence is not possible, because incidence is you'll call, is the proportion of new cases of a certain disease in a population.

02:16 And the major challenge in a case-control study is how to select the controls, how to do so without introducing new kinds of bias. Now in our lecture on bias, we cover this particular example, a 1981 study in the New England Journal of Medicine that showed an association between drinking coffee and getting pancreatic cancer. Now that was a case-control study, the cases were people with pancreatic cancer and the controls were people in the hospital that didn't have pancreatic cancer, but had other G.I. Issues, gastrointestinal issues. The problem of course is that those with gastrointestinal issues were less likely to have drunk coffee and so by comparing the two groups, we have an artificially exaggerated association between coffee drinking and pancreatic cancer. That was a case of selection bias.