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Systematic Reviews

The Basics

Formulating a PICO Question

PICO stands for Population, Intervention, Comparison and Outcome.


  • How would you describe this population? (age, race, sex, healthy, risk factors, previous or current ailments, current medications, etc.)
  • How is the condition defined? (symptoms, presence and/or severity of disease, diagnostic test, etc.)
  • Are there any patients who should be excluded from this population? (healthy patients, patients above or below a certain threshold, etc.)


  • What main intervention, prognostic factor, or exposure are you considering?
  • Are you interested in a drug treatment? Medical procedure? Surgical procedure? Diagnostic test?
  • For drug interventions, what is the dosage, frequency, and method of administration?


  • What is the main alternative to your intervention?
  • A different dosage of the same drug? Placebo or alternative drugs? Another medical or surgical procedure?
  • For diagnostic studies, is there a gold standard or another diagnostic tool with which to compare yours?


  • What are you hoping to accomplish, measure, improve, or affect?
  • Do you want to improve quality of life?
  • Are morbidity or mortality important issues to consider?
  • What are the harms of this treatment or test and its alternatives?


How will PICO help me search?

PICO is used to create a researchable question based on a clinical situation you have encountered. Based on your PICO question, you will identify keywords and/or subject terms to use in database searches. When starting your search, you will want to focus your keywords and subject terms on your P and I. For example:

Research Question: Does caffeine intake in children cause stunted growth?

P: children

I: caffeine intake

C: children with no caffeine intake

O: stunted growth

When starting the search, you would want to focus your search terms on children and caffeine.

A generic search string for this search may look like this:

(child* OR adolescen* OR teen* OR toddler* OR baby OR babies OR infan* OR youth*) AND (caffein* OR coffee* OR soda* OR tea*)


Boolean logic is a building block of many computer applications and is an important concept in database searching.  Using the correct Boolean operator can make all the difference in a successful search.


There are three basic Boolean search commands: AND, OR and NOT.

  • AND searches find all of the search terms.  For example, searching on dengue AND malaria AND zika  returns only results that contain all three search terms.  Very limited results.

  • OR searches find one term or the other.  Searching on dengue OR malaria OR zika returns all items that contain any of the three search terms.  Returns a large number of results.
  • NOT eliminates items that contain the specified term.  Searching on malaria NOT zika returns items that are about malaria, but will specifically NOT return items that contain the word zika.  This is a way to fine-tune results. Note:  sometimes AND NOT is used;  serves the same function as NOT.

Using Boolean Search with Exact Phrases

If you're searching for a phrase rather than just a single word, you can group the words together with quotation marks.  Searching on "dengue fever" will return only items with that exact phrase.  

When to use parentheses?

It's a lot like basic math.  (2 × 4) + 1 = 9   but 2 × (4 + 1) = 10

Think of your search in concepts, then put those concepts inside parentheses.  Different databases have different rules about combining searches.  To make sure you get the search you want, use parentheses - every database follows those rules. For example:

dengue OR malaria AND zika  can be interpreted as

  •  (dengue OR malaria) AND zika = articles about dengue or malaria, that also discuss zika.  Every item returned would mention zika, and either dengue or malaria.  As the zika virus has only recently been a serious issue, this would limit the number of results.
  • dengue OR (malaria AND zika) = every article about dengue, or those that discuss both zika and malaria.  Since dengue fever has been a concern for over 250 years, this search would yield different results.  Every item about dengue would be returned, as would those that discuss both of the other two.

Controlled vocabularies are searchable thesauri or collections of terms that are used to index, catalog, and search databases. Using a database's controlled vocabulary often helps provide better results, which is why it is important that complex search strategies use a combination of controlled vocabulary terms and keywords. Controlled vocabularies that you may run into while conducting systematic reviews may include MeSH, CINAHL Subject Headings, and the APA Thesaurus of Psychological Index Terms.


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