Dan Mayer Essential Evidence-based Medicine

case–control, orcross-sectional studies. These will be defined in more detail
in Chapter 6. In studies of etiology, the risk factor for a disease is the cause and
the presence of disease is the outcome. In other studies, the cause could be a
therapy for a disease and the effect could be the improvement in disease.
Studies of therapy or prevention tend to be randomized clinical trials,inwhich
some patients get the therapy or preventive modality being tested and others
do not. The outcome is compared between the two groups.
Studies of prognosis look at disease progression over time. They can be either
cohort studies or randomized clinical trials. There are special elements to
studies of prognosis that will be discussed in Chapter 33.
Studies of diagnosis are unique in that we are looking for some diagnostic
maneuver that will separate those with a disease from those who may have a
similar presentation and yet do not have the disease. Usually these are cohort,
case–control,orcross-sectional studies. These will be discussed in more detail
in Chapter 28.
There is a relationship between the clinical question and the study type. In
general the clinical question can be written as: among patients with a particular
disease (population), does the presence of a therapy or risk factor (intervention),
compared with no presence of the therapy or risk factor (comparison), change
the probability of an adverse event (outcome)? For a study of risk or harm, we
can write this as: among patients with a disease, does the presence of a risk factor,
compared with the absence of a risk factor, worsen the outcome? We can
also write it as: among patients with exposure or non-exposure to a risk factor,
are they more likely to have the outcome of interest? For therapy, the question
is: among patients with a disease, does the presence of an exposure to therapy,
compared with the use of placebo or standard therapy, improve the outcome?
The form of the question can help you perform better searches, as we will see in
Chapter 5. Through regular practice, you will learn to write better questions and
in turn, find better answers.
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