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What is a clinical trial? A clinical trial is a type of research study designed to evaluate the effect of an intervention on a health-related outcome in humans. The intervention can take many forms—medical therapies, behavior changes, or educational materials or websites. The outcome of interest can also be defined in a wide variety of ways—overall survival, disease progression, a body measure (eg, cholesterol level, weight), quality of life or well-being, behavioral change, or other factors. Clinical trials are done to evaluate new approaches to prevention, screening, diagnosis, or treatment.

There are many steps and components to ensure that such research is conducted rigorously, methodically, and in a logical fashion. This chapter briefly summarizes the following topics, as they relate to clinical trials: design, ethics, trial phases, strengths and weaknesses, and interpretation caveats. This chapter provides only a brief introduction to important concepts about understanding clinical trials and is meant to facilitate critical reading of the peer-reviewed literature. If you are planning to conduct clinical research or a clinical trial, we recommend that you seek out expertise from an epidemiologist or biostatistician, or pursue further in-depth education.


To better understand the design of a clinical trial, it is important to see how it fits into the context of other clinical epidemiology and research designs. There are many types of clinical research studies and different ways to categorize them. Four common designs are ecological studies, case–control studies, cohort studies, and clinical trials. To appreciate the pros and cons of a clinical trial, it is helpful to understand a little about the other types of study designs (see Figure 48–1).

Figure 48–1.

Common epidemiologic study designs.

Clinical Trials versus Other Epidemiologic Study Designs

What is the goal of epidemiologic studies? In general, the goal of epidemiologic research is to make causal inferences about what causes particular diseases or conditions in a given population. Several types of study designs are used for this purpose, each with strengths and weaknesses. We will summarize these briefly to understand better where clinical trials fit into the epidemiology toolbox, and how they have unique strengths and weaknesses.

First, there are some important concepts to understand when assessing a study design’s rigor: outcome, exposure, temporality and causation, confounding, and bias.

A. Outcome

An outcome is typically the disease or condition that we are interested in understanding. For example, this could be heart attacks, cancer, diabetes, or urinary tract infections. There are also different ways to measure disease outcomes. One can simply look at whether people developed the disease (yes or no) during a certain time period. Alternatively, one could measure the rate of how ...

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