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Systematic Review Resources: P(I)ECES of a Systematic Review: Identifying the Evidence

This guide is designed to help you get started on a systematic review and provide guidance on a wide variety of resources.

P(I)ECES of a Systematic Review : Identifying and Searching the Evidence

I: Identifying the Evidence

It is important when searching for evidence that search terms are referred back to your original PICO question. 

"Levels of Evidence" tables have been developed which outline and grade the best evidence. However, the review question will determine the choice of study design.

Secondary sources provide analysis, synthesis, interpretation and evaluation of primary works. Secondary sources are not evidence, but rather provide a commentary on and discussion of evidence. e.g. systematic review

Primary sources contain the original data and analysis from research studies. No outside evaluation or interpretation is provided. An example of a primary literature source is a peer-reviewed research article. Other primary sources include preprints, theses, reports and conference proceedings.

Levels of evidence for primary sources fall into the following broad categories of study designs (listed from highest to lowest):

  • Experimental: RTC's (Randomised Control Trials)
  • Quasi-experimental studies (Non-randomised control studies, Before-and-after study, Interrupted time series)
  • Observational studies (Cohort study, Case-control study, Case series) 

The Evidence Pyramid represents the hierarchy and availability of the different types of research evidence and studies. As you can see, Systematic Reviews and Meta-Analysis are the highest level of evidence, but the least populous of all the study types.

Based on information from Centre for Reviews and Dissemination. (2009). Systematic reviews: CRD's guidance for undertaking reviews in health care. Retrieved from



Searching for Prior Reviews

Why Search for Prior Review

Prior to starting a systematic review you need to find out if a systematic review has been done or is under way. Checking for prior reviews helps you determine if your review is unique or ensures that you aren't repeating a review that has been done or registered. There are several key  points to note: 

I.          Determine if your review is unique.You don't want to replicate a review if it has just been done!

a)         Is it out of date?

b)         Maybe it wasn't just done. Maybe it's a few years old.  Does it need to be updated?

II.         If your idea is taken, how can you modify it to make it unique?

            a)         Be careful in your selection criteria.  

            b)         Read the prior review very carefully.

For example, if you wanted to do a review on interventions to reduce adolescent pregnancy and (surprise, surprise) it's already been done, don't think that you can just throw in race or ethnicity and make it all brand new. 

Did they do an analysis by race/ethnicity?  Did they indicate there were even enough studies with that sort of a focus?

If a prior, recent review indicates the studies aren't there, then you could very well be wasting your time trying to do a review in an area in which there is little or no original research.

III.        Determine if a similar review exists which means you can:

a)         Evaluate the review

       Was the search strategy appropriate?

       Did they evaluate the quality of the studies or simply vote count?

       How well do they describe their methods?

Take note: Reporting not necessarily an indicator of quality; it could just be an indicator the author did not know to use reporting standards

IV.       What is different about your SR?

            a)         Theory/model, race/ethnicity, age, setting, etc.

Verifying if a systematic review has been done could save you a great deal of time (and heartbreak!) later.

Evidence Pyramid

The following image represents the hierarchy and availability of the different types of research evidence and studies. As you can see, Systematic Reviews and Meta-Analysis are the highest level of evidence, but the least populous of all the study types.




See Journal Databases

Types of Study Design

The following definitions are from the Glossary on the Centre for Evidence Based Medicine (CEBM), University of Oxford, website: 

  • Systematic Review
    An article in which the authors have systematically searched for, appraised, and summarised all of the medical literature for a specific topic.
  • Critically appraised topic
    A short summary of an article from the literature, created to answer a specific clinical question.
  • Randomized control clinical trial (RCT)
    A group of patients is randomised into an experimental group and a control group. These groups are followed up for the variables/outcomes of interest.
  • Cohort study
    Involves the identification of two groups (cohorts) of patients, one which did receive the exposure of interest, and one which did not, and following these cohorts forward for the outcome of interest.
  • Case-control study
    Involves identifying patients who have the outcome of interest (cases) and control patients without the same outcome, and looking to see if they had the exposure of interest.
  • Meta-analysis
    A systematic review which uses quantitative methods to summarise the results.