Study designs in medicine fall into two categories: studies in which subjects are observed, and studies in which the effect of an intervention is observed.
Observational studies may be forward-looking (cohort), backward-looking (case–control), or looking at simultaneous events (cross-sectional). Cohort studies generally provide stronger evidence than the other two designs.
Studies that examine patient outcomes are increasingly published in the literature; they focus on specific topics, such as resource utilization, functional status, quality of life, patient satisfaction, and cost-effectiveness.
Studies with interventions are called experiments or clinical trials. They provide stronger evidence than observational studies.
The single best way to minimize bias is to randomly select subjects in observational studies or randomly assign subjects to different treatment arms in clinical trials.
Bias occurs when the way a study is designed or carried out causes an error in the results and conclusions. Bias can be due to the manner in which subjects are selected or data are collected and analyzed.
Clinical trials without controls (subjects who do not receive the intervention) are difficult to interpret and do not provide strong evidence.
Each study design has specific advantages and disadvantages.
This chapter introduces the different kinds of studies commonly used in medical research. Knowing how a study is designed is important for understanding the conclusions that can be drawn from it. Therefore, considerable attention will be devoted to the topic of study designs.
If you are familiar with the medical literature, you will recognize many of the terms used to describe different study designs. If you are just beginning to read the literature, you should not be dismayed by all the new terminology; there will be ample opportunity to review and become familiar with it. Also, the glossary at the end of the book defines the terms used here. In the final chapter of this book, study designs are reviewed within the context of reading journal articles, and pointers are given on how to look for possible biases that can occur in medical studies. Bias can be due to the manner in which patients are selected, data are collected and analyzed, or conclusions are drawn.
CLASSIFICATION OF STUDY DESIGNS
There are several different schemes for classifying study designs. The one most relevant in clinical applications divides studies into those in which the subjects were merely observed, sometimes called observational studies, and those in which some intervention was performed, generally called experiments. This approach is simple and reflects the sequence an investigation sometimes takes. With a little practice, you should be able to read medical articles and classify studies according to the outline in Table 2–1 with little difficulty.
Table 2–1.Classification of study designs. ||Download (.pdf) Table 2–1. Classification of study designs.
Descriptive or case–series
Case–control studies (retrospective)
Causes and incidence of disease