Dan Mayer Essential Evidence-based Medicine

Searching the medical literature 35
Outcome
Mortality
Screening
Colorectal
neoplasms
Intervention
Population
Fig. 5.1 Venn diagram for
colorectal screening search.
Comparison is frequently
omitted in search strategies.
Developing effective information retrieval strategies
Having selected the most appropriate database one must develop an effective
search strategy to retrieve the best available evidence on the topic of interest.
This section will give a general searching framework that can be applied to any
database. Databases often vary in terms of software used, internal structure,
indexing terms, and amount of information that they give. However, the principles
behind developing a search strategy remain the same.
The first step is to identify the key words or concepts in the study question. This
leads to a systematic approach of breaking down the question into its individual
components. The most useful way of dividing a question into its components is
to use the PICO format that was introduced in Chapter 2. To review: P stands
for the population of interest; I is the intervention, whether a therapy, diagnostic
test, or risk factor; C is the comparison to the intervention; and O is the outcome
of interest.
A PICO question can be represented pictorially using a Venn diagram. As an
example, we will use the following question: What is the mortality reduction in
colorectal cancer as a result of performing hemoccult testing of the stool (fecal
occult blood test, FOBT) screening in well-appearing adults? Using the PICO format,
we recognize that mortality is the outcome, screening with the hemoccult
is the intervention, not screening is the comparison, and adults who appear well
but do and don’t have colorectal neoplasms is the population. The Venn diagram
for that question is shown in Fig. 5.1.
Once the study question has been broken into its components, they can be
combined using Boolean logic. This consists of using the terms AND, OR, and
NOT as part of the search. The AND operator is used when you wish to retrieve
those records containing both terms. Using the AND operator serves to narrow
your search and reduces the number of citations recovered. The OR operator
is used when at least one of the terms must appear in the record. It broadens
the search, should be used to connect synonyms or related concepts, and will
increase the number of citations recovered. Finally, the NOT operator is used to