Case-Control and Cohort Studies

This post is apropos of an article we recently reviewed in journal club, which used both methodologies.  You can listen to a recording of the remarks we made at the time by following this link, or read on.

The first thing to understand about cohort and case-control studies is that they're both observational as opposed to experimental studies.  This is easy to understand in principle, but it is stunning how often people will try to fool you (using techniques like propensity-score matching, discussed in the last section of the post on our GI Journal Club) into thinking that an observational study is actually experimental.

The second thing to understand is that the two represent very different levels of evidentiary quality, which is obvious once you understand their methodologies.

In a prospective cohort study, a group (or cohort) are identified prior to the development of the outcome(s) of interest.  The presence or absence of putative risk factors (i.e. "exposures") thought to be related to the outcome are then studied in relation to the development of the outcome(s) the investigator wants to know about.  A classic and famous example is the Framingham Heart Study, in which a large cohort were identified and closely studied prior to the development of heart disease, and from which many valuable associations between risk factors like smoking, cholesterol and hypertension and cardiovascular outcomes have emerged.  Prospective cohort studies are considered to be the "gold standard" of observational research, in the same way that the RCT is considered the "gold standard" of experimental research.

Case-control studies, on the other hand, come at the problem from the other end (indeed, some have even proposed they be called "trohoc" studies, because they are cohort studies done backwards).  A group of cases are selected in which the outcome of interest has already developed, and then they are compared to people who appear otherwise similar but do not have the outcome in order to determine the strength of association between the outcome and exposures thought to be risk factors. Some of the earliest work associating tobacco consumption with lung cancer was done through case-control studies.  The primary weakness of case-control studies is that they are by definition retrospective, and therefore subject to various forms of recall bias.  Case-control studies are not considered the "gold-standard" of anything.

An important point to understand is that cohort studies can generate infomation about the incidence of a condition, which allows you to calculate the relative risk of the outcome associated with a specific exposure.  However, in a case-control study the investigator decides how many subjects have the outcome, because they select the cases - so there is no "incidence," and the strength of association can only be expressed by relatively indirect methods like the odds ratio.