- 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
- 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
- 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
- Clinical trials without controls (subjects who do not receive
the intervention) are difficult to interpret and do not provide
- Each study design has specific advantages and disadvantages.
This chapter introduces the different kinds of studies commonly
used in medical research. Because we believe that knowing how a
study is designed is important for understanding the conclusions
that can be drawn from it, we have chosen to devote considerable
attention 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 we use 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.
There are several different schemes for classifying study designs.
We have adopted one that divides studies into those in which the
subjects were merely observed, sometimes called observational
and those in which some intervention was performed,
generally called experiments.
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
with little difficulty.
Table 2–1. Classification
of Study Designs. |Favorite Table|Download (.pdf)
Table 2–1. Classification
of Study Designs.
|I. Observational studies|
|A. Descriptive or case–series|
|B. Case–control studies (retrospective)|
|1. Causes and incidence of disease|
|2. Identification of risk factors|
|C. Cross-sectional studies, ...|