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Systematic Reviews

Why is grey literature important?

It will depend on the purpose and scope of the review whether or not to include grey literature in your search, however, there are a number of reasons why it may be important to include in your review:  

  • Grey literature can provide further sources of evidence for your review 
  • It can provide a more rounded view of your field of research with access to different perspectives 
  • It can help balance publication bias e.g. often negative and neutral research results are not published by conventional means 
  • It can be a source of raw data 
  • It can provide more currency to your review through access to the most up-to-date and pre-publication material

What is grey literature

Definitions of grey literature vary, but it is generally accepted to mean any literature that is not formally published in conventional and established formats. Examples would include: 

  • Conference proceedings 
  • Dissertations 
  • Working papers 
  • Government reports 
  • Research group reports 
  • Organisation's websites 
  • Pre-print material

Challenges of using grey literature

Searching for grey literature can be a time-consuming process, as you may need to search in many different places to find relevant material which may be difficult to locate and access. This makes searching grey literature systematically a challenging process.

Depending on your topic and available time, you may decide to search all or a limited range of grey literature. 

You will also have to carefully assess the quality and validity of the information located when searching this type of material. 

How to search for grey literature

Methods of searching for grey literature may include: 

  • Grey literature database searches e.g. Zetoc, OpenGrey, Scopus, Ethos etc. 
  • Web searches 
  • Hand-searching 
  • Targeted searching, e.g. relevant organisations 
  • Correspondence with field experts 

Using grey literature in your review

As part of your overall search methodology or planning, you may wish to clearly articulate how you intend to approach searching for grey literature. You may wish to incorporate this into your search plan. Your grey literature search plan should aim to include how and where you intend to search and the rational behind this.

You may also need to record grey literature searching and results in a different way, and separately from other database searching.

Searching multiple source formats and platforms or employing alternative methods of data collection (e.g. correspondence with experts) means it will not be possible to replicate the same kind of search strategy you would use for mainstream sources.

You will also need to consider how you will record the information you find, as not all grey literature sources may be able to be integrated with referencing software.

Database / Web Site Date Search Terms URL Results
3ie Database of Impact Evaluations 09/05/11 Melaria



African Index Medicus 05/05/11 Search on :  "MALARIA" or "MALARIAE" or "PLASMODIUM" or "MOSQUITO" or "MOSQUITOES" [Key Word] and  "BEDNETS" or "NET" or "NETTING" or "CURTAINS" or "BEDDING" [Key Word] 25
Google Scholar 26/05/11

STRATEGY 1: allintitle: malaria AND (bednet OR bednets OR net OR nets OR curtain OR curtains OR netting OR ITNs) AND (own OR ownership OR possession OR use OR usage OR uses OR cost OR afford OR delivery OR subsidy OR subsidize OR purchase OR price OR utilize OR utilization OR voucher OR vouchers OR campaign OR program OR programs OR distribution) 

STRATEGY 2: Search: allintitle: malaria AND (bednet OR bednets OR net OR nets OR curtain OR curtains OR netting OR ITNs) AND (random OR randomly OR randomized OR controlled OR trial OR intervention OR clinical OR "pre‐post test" OR "time interrupted" OR "before and after" OR RCT OR CBA OR CCT OR ITS) 98

This table is an extract from Appendix 2 of a Cochrane Review on Strategies to increase the ownership and use of insecticide-treated benders to prevent malaria.