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Epidemiological Studies Covered on the Physician Assistant Exam

By Barry Schoenborn, Richard Snyder

For the Physician Assistant Exam (PANCE), you should be aware of the differences among the types of epidemiologic studies. The first step is to recognize whether the study is a retrospective study or a prospective study:

  • Retrospective: A retrospective study means that you’re looking into the past to figure out what happened. For example, a retrospective study may involve looking at patient charts. Or it may involve searching medical databases and registries, such as the Cochrane Library, for specific conditions.

  • Prospective: A prospective study means that you’re looking to the future to see whether a specific outcome will happen. A classic example of a prospective study is the large clinical trials to evaluate a medication and its effects on treating a medical condition.

Randomized controlled trials

Clinical drug trials are often randomized controlled studies, the best kind of clinical research trials. The randomized controlled study includes two groups of people: one control and one intervention, or variable (the group being tested). The distribution of participants between the control group and intervention group is random and avoids allocation bias.

This type of study is the best because it’s usually double-blinded — in plain English, this means that both the investigator and those being tested walk around wearing big goggles. Seriously, double-blinded means the study is designed so neither the investigator nor the participant knows upfront which treatment is being given to which person. This blinding prevents bias. Bias, of course, is a bad thing in the research community.

Case-control studies

A case-control study is a type of retrospective analysis where you’re comparing a control group (those who are healthy) with another group that has a certain medical condition. This study is purely an observational analysis and depends on looking up patient data. As a researcher, you’d look at some factor or characteristic in both groups to see whether there’s any link between the specified characteristic and the disease.

For example, in a case-control study, you may compare people who’ve had myocardial infarctions (MI) with those who haven’t, comparing the prevalence of diabetes in each group. You’d find that diabetes is more prevalent in the MI group. Of course, diabetes increases the risk of an MI.

Cohort studies

A cohort study follows a group of people (a cohort) over a period of time, sometimes years. Here, you first identify persons who’ve been exposed to Factor X. You follow these people over a defined time interval and see whether they develop either certain symptoms or the actual condition you’re studying.

For example, perhaps you’re watching a select population who may have been exposed to a pesticide during childhood. They may have had different levels of exposure, but you watch them for the development of any symptoms.

Cross-sectional studies

If a cohort study is following people over time, a cross-sectional study is just the opposite: It’s looking at a medical condition and the population at the same time. In other words, a cross-sectional study tries to tell you the incidence or prevalence of a certain medical condition, depending on what you’re looking for.

Say you wanted to know the incidence of people with three eyeballs within a population. If you were to find out that many of the children who developed three eyeballs had been exposed to a certain pesticide, then you’d need to study this population specifically. Maybe someday you’ll do this study. We’ll keep an eye out for you.

You want to study the potential link between exposure to a pesticide and the development of three eyeballs. You have identified a group of people who have been exposed to a certain pesticide. You decide that you want to watch them over a period of five years to see whether they develop a third eyeball. Which type of study would you be conducting?

(A) Randomized controlled study

(B) Case-control study

(C) Cohort study

(D) Retrospective study

(E) Cross-sectional study

The correct answer is Choice (C). This is an example of a cohort study. You’re going to follow this same group of people over time to see whether they develop that third eyeball. For a randomized controlled study, Choice (A), you’d need to have two groups of people: one group who had been exposed to a certain measure of the pesticide and one group who hadn’t.

Neither the investigator nor the participants would know whether a particular participant had been exposed to the pesticide or to a harmless substance. A case-control study, Choice (B), would involve looking at people who developed three eyeballs and people who hadn’t and comparing the exposure to pesticides in each group.

The cross-sectional study, Choice (E), would be looking the exposure of a pesticide and the development of three eyeballs that had already occurred over a given period of time. You aren’t following people over time with this type of study.