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Systematic Reviews

Sources of information for systematic review

Literature search for systematic review must be as comprehensive as possible to minimise publication bias and to ensure the comprehensiveness of the review.  The souces of information include both published and unpublished materials.   The core databases to search based on Cochrane Handbook are:

  1. MEDLINE
  2. Embase
  3. The Cochrane Library (The Cochrane Central Register of Controlled Trials(CENTRAL))

In addition to the above, depending on the topic, scope and purpose of the systematic review, other databases may be searched.  This includes subject specific databases (e.g. CINAHL, ERIC, PsycINFO, etc.), clinical trials registries and grey literature and so on.   For more details, please refer to Guidance on literature searching mentioned below:

Guidance on literature searching

Cooper, C., Booth, A., Varley-Campbell, J. et al. Defining the process to literature searching in systematic reviews: a literature review of guidance and supporting studies. BMC Med Res Methodol 18, 85 (2018). https://doi.org/10.1186/s12874-018-0545-3

Table 2 The order of literature search methods as presented in the guidance documents

Step

The CRD Handbook

The Cochrane Handbook

Collaboration for Environmental Evidence

Joanna Briggs Institute Reviewers' Manual

IQWiG Methods Resources

Systematic reviews in the Social Sciences: A Practical Guide

EUnetHTA

Campbell Handbook

Developing NICE guidelines: the manual

1

Searching electronic databases

Searching bibliographic databases

Searching online literature databases and catalogues

Databases (development of search strategies, phase one)

Bibliographic databases (1.search for primary literature. 2. search for SRs)

Databases

Bibliographic databases

Bibliographic databases (1. subject databases. 2. general databases)

No list of search methods but guidance distinguishes between database searching (first) and supplementary searching (second)

2

Scanning references lists of relevant studies

Handsearching

Searching websites of organisations and professional networks

Database searching (phase two)

Search in trial registries

Grey literature

Study registries

Conference proceedings and meeting abstracts

3

Handsearching of key journals

Conference abstracts or proceedings

Searching the world-wide web

Review reference lists

Clinical practice guideline databases and providers

identifying on-going research

Searching for unpublished company documents

Existing review and publication reference lists

 

4

Searching trials registers

Other reviews

Searching bibliographies of key articles/ reviews

Handsearching

Requests to manufacturers

Theses

Regulatory documents

Web searching

 

5

Contacting experts and manufactures

Web-searching

Contacting key individuals who work in the area

 

Other data sources

Conference proceedings

Queries to authors

Unpublished studies

 

6

Searching relevant internet resources

Unpublished and on-going studies (inc. author contact)

Citation searches for key papers/ included papers

 

 

Citation searching

Further search techniques

On-going studies

 

7

Citation searching

 

 

 

 

Searching the web

 

Institutional repositories

 

8

Using a project website to canvas for studies

 

 

 

 

contact with experts

 

handsearching

 

9

 

 

 

 

 

Trials registers

 

 

 

Kugley, S., Wade, A., Thomas, J., Mahood, Q., Jørgensen, A.-M.K., Hammerstrøm, K. and Sathe, N. (2017), Searching for studies: a guide to information retrieval for Campbell systematic reviews. Campbell Systematic Reviews, 13: 1-73. https://doi.org/10.4073/cmg.2016.1

Appendix V. MEC2IR literature search conduct standards

Item No. Status Item Name Standard Rationale and elaboration
24 Mandatory Planning the search Refer to the white paper by Hammerstrom, Wade, and Jorgensen (2010) to ensure that all relevant databases have been properly searched. Searches for studies should be as extensive as possible to reduce the risk of publication bias and to identify as much relevant evidenceas possible. There is no minimum set of databases to search, but reviewers should consider consulting with a research retrieval specialist to avoid unnecessary duplication of effort.
25 Highly desirable Searching specialist bibliographic databases Search appropriate national, regional, and subject specific bibliographic databases. Searches for studies should be as extensive as possible to reducethe risk of publication bias and to identify as much relevant evidence as possible. Databases relevant to the review topic should be covered (e.g., ERIC for educational interventions, PsycINFO for psychological interventions), and regional databases (e.g. LILACS) should be considered.
26 Mandatory Searching for different types of evidence If the review has specific eligibility criteria around study design to address adverse effects, economic issues, or qualitative research questions, undertake searches to address them. Sometimes different searches will be conducted for different types of evidence, such as for non-randomized studies for addressing adverse effects, or for economic evaluation studies.
27 Mandatory (if applicable) Searching trials registers When relevant, search trials registers and repositories of results, where relevant to the topic through ClinicalTrials.gov, meta REGISTER, the WHO International Clinical Trials Registry Platform (ICTRP) portal, and other sources as appropriate. When relevant, searches for studies should be as extensive as possible to reduce the risk of publication bias and to identify as much relevant evidence as possible. Although ClinicalTrials.gov is included as one of the registers with in the WHO ICTRP portal, it is recommended that both ClinicalTrials.gov and the ICTRP portal are searched separately  due to additional features in ClinicalTrials.gov.
28 Mandatory Searching for grey literature Search relevant grey literature sources such as reports/dissertations/theses databases and databases of conference abstracts. Searches for studies should be as extensive as possible to reduce the risk of publication bias and to identify as much relevant evidence as possible.
29 Mandatory Searchingwithin other reviews Search with in previous reviews on the same topic. Searches for studies should be as extensive as possible to reduce the risk of publication bias and to identify as much relevant evidence as possible.
30 Mandatory Searching reference lists Check reference lists in included studies and any relevant systematic reviews identified. Searches for studies should be as extensiveas possible to reduce the risk of publication bias and to identify as much relevant evidenceas possible.
31 Highly desirable Searching by contacting relevant individuals and organizations Contact relevant individuals and organizations for in formation about un published orongoing studies. Searches for studies should be as extensive as possible to reduce the risk of publication bias and to identify as much relevant evidenceas possible. It is important to identify on going studies, so that when a review is updated, these can be assessed for possible inclusion.
32 Mandatory Structuring search strategies for bibliographic databases Inform the structure of search strategies in bibliographic databases around the main concepts of the review, using appropriate elements from PICO and study design. In structuring the search, maximize sensitivity whilst striving for reasonable precision. Ensure correct use of the AND and OR operators.

Inappropriate or inadequate search strategies may fail to identify records that are included in bibliographic databases. Expertise may need to be sought, in particular from an Information retrieval specialist. The structure of a search strategy should be based on the main concepts being examined in a review. In electronic bibliographic databases, a search strategy to identify studies for a Campbell Review will typically have three sets of terms: 1) terms to search for the population of interest; 2) terms to search for the intervention(s) evaluated; and 3) terms to search for the types of study designs to be included. There are exceptions, however.

For instance, for reviews of complexinte inventions, it mayv be necessary to search only for the population or the intervention. Within each concept, terms are joined together with the Boolean ‘OR’ operator, and the different concepts are combined with the Boolean ‘AND’ operator. The ‘NOT’ operator should be avoided where possible to avoid the danger of inadvertently removing from the search set records that are relevant.

33 Mandatory Developing search strategies for bibliographic databases Identify appropriate controlled vocabulary (e.g. MeSH, Emtree, including ‘exploded’ terms) and free-text terms (considering, for example, spelling variants, synonyms, acronyms, truncation, and proximity operators), and tailor the search strategy to each specific database. Inappropriate or inadequate search strategies may fail to identify records that are included in bibliographic databases. Search strategies need to be customised for each database. It is important that MeSH terms are ‘exploded’ wherever appropriate, in order not to miss relevant articles. The same principle applies to EMTREE when searching EMBASE and also to a number of other databases. The controlled vocabulary search terms are different for each electronic database, and thus search strategies must be tailored to each database. To be as comprehensive as possible, it is necessary to include a wide range of free-text terms for each of the concepts selected. This might include the use of truncation and wild cards. Developing a search strategy is an iterative process in which the terms that are used are modified, based on what has already been retrieved.
34 Highly desirable Using search filters Use specially designed and tested search filters where appropriate (such as the Cochrane Highly Sensitive Search Strategies for identifying randomized trials in MEDLINE), but do not use filters in pre- filtered databases (e.g. do notuse a randomized trial filter in CENTRAL or a systematic review filter in DARE). Search filters should be used with caution. They should be assessed not only for the reliability of their development and reported performance but also for their current accuracy, relevance, and effectiveness given the frequent interface and indexing changes affecting databases.
35 Mandatory Restricting database searches Justify the use of anyrestrictions in the search strategy on publication date, publication format, or language. Date restrictions in the search should only be used when there are date restrictions in the eligibility criteria for studies. They should be applied only if it is known that relevant studies could only have been reported during a specific time period, for example if the intervention was only available after a certain time point. Searches for updates of reviews might naturally be restricted by date of entry into the database (rather than date of publication) to avoid duplication of effort. Publication format restrictions (e.g. exclusion of letters) should generally not be used in Campbell reviews, since any information about an eligible study may be of value.
36 Mandatory Documenting the search process Document the search process in enough detail to ensure that it can bereported correctly in the review/update. Include the month and year the search began and ended for future replicability. The search process (including the sources searched, when, by whom, and using what terms) needs to be documented in enough detail throughout the process to ensure that it can be reported correctly in the review, to the extent that all the searches of all the data bases are reproducible.
37 Highly desirable Rerunning searches Rerun or update searches for all relevant databases within 12 months before publication of the review or review update, and screen the results for potentially eligible studies. The published review should be as up to date as possible. The search should be rerun close to publication, if the initial search date is more than 12 months (preferably 6 months) from the intended publication date, and the results screened for potentially eligible studies. Ideally the studies should be fully incorporated. If not, then the potentially eligible studies will need to be reported, at a minimum as a reference under ‘Studies awaiting classification’ or ‘Ongoing studies’.
38 Highly desirable Incorporating findings from rerun searches Incorporate fully anystudies identified in the rerun or update of the search with in 12 months before publication of the reviewor review update. The published review should be as up to date as possible. After the rerun of the search, the decision whether to incorporate any new studies fully into the review will need to be balanced against the delay in publication.