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Evidence-Based Medicine: Searching Techniques

This is a libguide for praticing evidence-based medicine.

Translating PICO question into search strategy

You have broken down the clinical question into its PICO components and corresponding search terms. Next, you can combine search terms into a variety of search strategies, adapted to each resources. The top EBM resources is that you can keep searches simple because the databases are highly selective and relatively small. One or two search terms for the population or problem and for your intervention or exposure will find most relevant topics. For example, if you are interested in the diagnosis for schizophrenia, simply searching with the terms “schizophrenia” in summaries (eg, UpToDate) will usually suffice. In contrast, searching the large primary studies database such as PubMed usually requires more specific and structured searches.To find the evidence you need in large databases, your search terms should closely relate to the components of your PICO question.

Clinical question: In patients with suspected schizophrenia, is magnetic resonance imaging (MRI) compared with computed tomography (CT scan) more accurate in diagnosing schizophrenia?

PICO Components

Potential Search Terms

Examples of Search Strategies (PubMed)

Examples of Search Strategies (Ovid Medline)


patients with suspected schizophrenia



exp Schizophrenia/ OR



magnetic resonance imaging (MRI)

magnetic resonance imaging (MRI)

Magnetic Resonance Imaging OR MRI  OR NMR

exp Magnetic Resonance Imaging/ OR

"Magnetic Resonance".ti,kw.


(MRI OR NMR).ti.


computed tomography (CT scan)

computed tomography (CT scan)

"Tomography, X-Ray Computed"[Mesh] OR “computed tomography” OR CT

exp Tomography, X-Ray Computed/


("comput* tomography" OR CT).ti,kw.


Accurate diagnosis



exp Diagnosis/



Note: ti,kw means searching the terms in title and keyword fields.

Boolean operators AND or OR are usually used when combining search terms. OR finds studies containing either of the specified words/phrases, and broadens your search. AND finds studies containing both specified words/phrases, and narrows your search. The AND operator is useful when attempting to link different concepts together (e.g., PICO question). Below is the Ovid Medline search strategy for the example clinical question:

          #     Searches     
          1     exp Schizophrenia/     
          2     Schizophreni*.ti,kw.     
          3     1 or 2     
          4     exp Magnetic Resonance Imaging/     
          5     "Magnetic Resonance".ti,kw.     
          6     (MRI or NMR).ti.     
          7     4 or 5 or 6     
          8     exp Tomography, X-Ray Computed/     
          9     ("comput* tomography" or CT).ti,kw.     
         10     8 or 9     
         11     exp Diagnosis/
         12     diagnos*.ti,kw.     
         13     11 or 12     
         14     3 and 7 and 10 and 13

Broad vs Narrow Searches

If you initially found little evidence, you can broaden your search (eg, increase its sensitivity) by adding synonyms for each concept or using truncated terms (eg, schizophreni* will retrieve schizophrenia and schizophrenic and variant spelling( many other similar terms with different endings). Conversely, if your initial search retrieved too many citations to be screened, you can narrow your search (eg, increase its specificity) by linking more PICO components with “AND” or by adding limits (publication date, age group, etc.) and methodological filters (eg, Clinical Queries;). More sophisticated approaches include entering PICO components sequentially according to their importance to obtain a manageable number of articles in large databases, such as PubMed.

Being very specific in searches will ensure that the answer we get is applicable to the patient. However, it is a double-edged sword. We may fail to find any studies that meet all PICO elements. The solution is to start with specific PICO components but be ready to remove specifications to find a relevant article. For example, we may be ready to remove the "gender", "age group" and other limits when we think it is likely that those specifications/characteristics won't make any difference in clinical outcomes interested.

Searching the Medical Literature Is Sometimes Fruitless

The medical literature will not be helpful when no feasible study design or measurement tools exist that investigators could use to solve a clinical issue. Your search also will not be successful if no one has conducted and published the necessary study. Before embarking on a search, carefully consider whether the yield is likely to be worth the time.

Get help from librarians

Because of the complexity and interconnections of medical databases, some searches simply require the help of information specialist. Collaboration with librarians who are knowledgeable about evidence based practice is essential to get efficient answer to clinical questions.  With librarians, you may talk about your clinical questions and formulated PICO questions, understand evidence-based information resources and structure and searching for different databases, and discuss the search keywords, subject headings and search strategies. Ask the librarians for Literature search help.