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Systematic Reviews

Formulate Your Question

​​A clear, specific, and answerable question is essential to a successful systematic review. A well formulated question will help determine your protocol and search strategy, and help you to find relevant and valid information quickly.


As you develop your question, PICO is a useful tool that can help you define the question's core concepts. There are four components that typically make up a clinical question:

  • Patient, problem, or population
  • Intervention
  • Comparison intervention (if applicable)
  • Outcomes

You may also see this framework referred to as PICO(T), with T referring to time.

Here's an example of a clinical question broken down using PICO:


"In patients with heart failure from dilated cardiomyopathy who are in sinus rhythm..."


"...would adding anticoagulation with warfarin to standard heart failure therapy..."


"...when compared with standard therapy alone..."


"...lead to lower mortality or morbidity from thromboembolism? Is this enough to be worth the increased risk of bleeding?"

The Cochrane Handbook suggests the following factors to consider when using PICO to develop your question:





Factors to consider for Patient/Problem/Population

  • How is the disease/condition defined?

  • What are the most important characteristics of these patients/this population?

  • Are there relevant demographic factors (age, sex, ethnicity)?

  • What is the setting (hospital, community)?

  • Who makes the diagnosis?

  • Are there other types of people who should be excluded from the review (because they are likely to react to the intervention in a different way)?

  • How will studies involving only a subset of relevant participants be handled?





Factors to consider for Interventions

  • What are the interventions of interest (experimental and comparison intervention)?

  • Does the intervention have variations (dosage/intensity, mode of delivery, personnel who deliver it, frequency of delivery, duration of delivery, timing of delivery)?

  • Are all variations to be included (for example is there a critical dose below which the intervention may not be clinically appropriate)?

  • How will trials including only part of the intervention be handled?

  • How will trials including the intervention of interest combined with another intervention (co-intervention) be handled?





Factors to consider for Outcomes

  • What outcomes are essential for decision-making (usually have an emphasis on patient-important outcomes)?

  • What outcomes would the reviewbe likely to be able to address if sufficient studies are identified, in order to reach a conclusion about the effects (beneficial and adverse) of the intervention(s)?

  • What outcomes would be useful for explaining effects?

  • Cover potential as well as actual adverse effects.

  • Consider outcomes relevant to all potential decision makers, including economic data.

  • Consider the type and timing of outcome measurements.

Higgins JPT, Green S (editors). Cochrane Handbook for Systematic Reviews of Interventions Version 5.1.0 [updated March 2011]. The Cochrane Collaboration, 2011.

Inclusion and Exclusion Criteria

Once you have a question, you can use those PICO components to help define your inclusion and exclusion criteria.

Inclusion Criteria are everything that a study must have to be included in the review. 

Exclusion Criteria are anything that would make a study ineligible to be included in the review. 

It is important to have these criteria clearly defined before beginning your search to ensure that your selection process is thorough, consistent, and reproducible, and focuses on studies applicable to the research question..

In addition to the factors associated with Populations, Interventions and Comparitors, and Outcomes, other parameters along which inclusion criteria could be set include:

  • Study design (Randomized controlled trials? Cohort studies? Case-Control Studies? etc.)
  • Date (Was it published sufficiently recently? Is the technology used outdated? etc.)

Scoping the Question

It is not generally possible to formulate an answerable question and determine appropriate inclusion criteria for a review without some knowledge of the existing research relevant to the question. You may need to conduct some preliminary research to help you develop a question that is viable for a systematic review.

Performing preliminary research on a topic can help you:

  • Explore the extent and nature of existing literature.
  • Identify gaps and uncertainties that might be addressed by a systematic review.
  • Understand the terms and concepts used in relevant literature.
  • Help identify appropriate parameters for the review (PICO).
  • Identify the potential scope of a systematic review.

Your research question needs to be specific enough to get a meaningful conclusion, but broad enough to have enough literature to analyze.

Prior investigation can help you assess whether a systematic review on a given question is justified. It is necessary to check whether there are already existing or ongoing reviews on your topic. Doing so may help you in choosing or refining your question.

Find current/on-going systematic reviews with:

Find published systematic reviews with:

If another study answers a similar question, you may want to consider a new focus or study type, but check a couple of things first:

  • How long ago was the review published?

If it has been a long time, another systematic review might be needed to reflect advances in the field.

  • How much new research has been published?

Regardless of the length of time, if a lot of new research has been published that significantly adds to and/or changes the existing body of evidence, then a new systematic review might be needed.

​If you choose to update an existing systematic review, this article can provide some guidance: