One step up from cross sectional studies, in terms of value of evidence in hypothesis testing, would be case control studies. These are studies where the comparison groups (case and control) are chosen for the outcome under study. Choosing where to find the subjects for both the group with the condition under study and that without the condition is critical. Sometimes the disease status is established with the use of laboratory tests or determination of a panel of experts. Many case control studies will take a sample from hospitalized patients, choosing those with the study condition and the non-case group from hospitalized subjects. These studies are retrospective with a backward direction to ascertain a related attribute or past event that is correlated with the disease under study. The research may have more than one comparison group.
The purpose of these studies is to discover a risk factor that might be correlated with the condition under study. One case control study will not prove that something is a risk factor. Care must be taken to identify any confounding factors that might be in the causal pathway. For example, there is a correlation between the use of alcohol and the use of tobacco. Depending on whether or not smoking is being investigated as a risk factor, it will be necessary to understand the use of alcohol in the two groups to determine if it is a risk factor or a confounding factor. This can be very difficult and statistics alone may not accomplish this. The analysis must involve critical thinking skills about the known or suspected risk factors and confounding factors.
Sometimes there are data sets out there that can be used to run analyses that have not been gathered for that purpose. If that is the case, the conclusions can be wildly inappropriate. When reliable investigators conduct case control studies they design them to obtain good information that will either provide a new avenue for hypothesis generation or that will confirm previous research. It is important to consider the statistical properties of the analysis that will be conducted with the gathered data. Did the study authors gather the data for the purpose identified in the publication? The use of sample size calculation will ensure the study is sized appropriately to detect a correlation for the risk factors with the condition under study.
To judge how much weight to put on information published from a case control study, it is important to critically analyze how the populations were identified. If it was self-reported by the subjects were there qualifying questions to determine the accuracy? If it was based on diagnosis by medical professionals, was this verified. Is it possible that some in the control group also had the condition but had not been diagnosed? Also, some thought should be given to the determination of the risk factors being studied. Would a reasonable person answer the questions truthfully? Can we believe self-report for these conditions? Was the control population biased? If hospital-based controls were used, were they appropriate? If determination of the disease status involved x-rays was the radiologist blinded?
It is not always easy to determine the answers to these questions when reading the results of a case control study analysis. This is where your critical thinking skills will help you out. Is it reasonable to believe that these determinations are unbiased? Are there other studies with similar results so the mass of evidence is in agreement? Many of the published research articles today are from case control studies. These are the appropriate design for many outcomes, especially those that are rare. These designs are also used where there is a long latent period or duration of expression. These studies are usually less expensive than cohort or cross-sectional studies and are subsequently the largest portion of published research, as a rule. As both the occurrence of disease and the risk factors occur in the past, it is difficult to determine which came first, which can make the observed association stronger than the actual association. Carefully done case control studies still contribute a lot of information in the study of diseases.
