In this recent blog entry, we discussed the hierarchy of evidence and briefly touched on the different types of weak evidence. Today I would like to begin to review stronger evidence as we move up the chart.
After opinions, case reports, and case series we have case control studies. I know, all the terminology gets confusing. A case control study is a retrospective study of patients who already have a certain condition compared with control subjects.
That almost reads like a foreign language. The best way to understand this is with an example. Let's say that we are setting up a case control study on lung cancer. We would compare patients who already have lung cancer with patients who don't have lung cancer. We would ask the patients with lung cancer how much they smoked in the past. These answers would be compared to the answers of the control patients who do not have lung cancer.
Case control studies can be somewhat less reliable than stronger forms of evidence like randomized clinical trials or cohort studies. This is because a statistical relationship between two conditions does not mean that one condition actually caused the other. For instance, lung cancer rates are higher for people without a college education (who tend to smoke more). But this does not mean that you can reduce the risk of lung cancer just by getting a college education.
Our next article will discuss cohort studies. I am sure that all of you are elated with joy [/sarcasm].