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Systematic review guide

A step by step guide to doing a systematic review

Using grey literature: AACODS

Remember, when you find grey literature resources, they have not been peer reviewed so make sure that you critically appraise them before including them in your review. One of the most popular ways to do this is using the AACODS checklist (pdf), which was created by Jess Tyndall. Its flexibility means that it can be applied to a wide variety of grey literature resources.

AACODS (Authority, Accuracy, Coverage, Objectivity, Date, Significance)

  • Authority: Who is responsible for the content and are they reputable?
  • Accuracy: Is the document supported by credible sources and was any data collection explicit and appropriate?
  • Coverage: Does the document clearly state the population their content covers and any limits?
  • Objectivity: Is there bias? Is it easily detected?
  • Date: Can you find a date for it? 
  • Significance: Does the document add something meaningful or unique to the research?

Reporting grey literature in your review

The following information is taken from this article PRISMA-S: an extension to the PRISMA Statement for Reporting Literature Searches in Systematic Reviews explaining the PRISMA-S checklist. It is a useful resource to refer to when looking at how you should present your review. 

Trial Registers

Even though results from trial registers are officially grey literature, they are treated differently from other resources. You include the number of results you found in the registries in the same box of the PRISMA flow diagram where you are recording your results from databases. In your methodology you should list the name of each study registry you searched and include a link or citation to it. 

Other grey literature resources

You will record the number of grey literature resources, and where you found them, on the right hand side of your PRISMA flow diagram. In your methodology, you should make it clear how you searched for the grey literature. Some examples (taken from the article PRISMA-S: an extension to the PRISMA Statement for Reporting Literature Searches in Systematic Reviews) include:

“We also searched the grey literature using the search string: “public attitudes” AND “sharing” AND “health data” on Google (in June 2017). The first 20 results were selected and screened.” [60]"

“The grey literature search was conducted in October 2015 and included targeted, iterative hand searching of 22 government and/or research organization websites that were suggested during the expert consultation and are listed in S1 Protocol. Twenty two additional citations were added to the review from the grey literature search.” [61]

“To locate unpublished studies, we searched Embase [via Embase.com] for conference proceedings since 2000 and hand-searched meeting abstracts of the Canadian Conference on Physician Health and the International Conference on Physician Health (2012 to 2016).” [62]

The article cited on this page is Rethlefsen, M.L., Kirtley, S., Waffenschmidt, S. et al. PRISMA-S: an extension to the PRISMA Statement for Reporting Literature Searches in Systematic Reviews. Syst Rev 10, 39 (2021). https://doi.org/10.1186/s13643-020-01542-z