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Jump to navigation. Chapter 4: Searching for and selecting studies. Cochrane Handbook for Systematic Reviews of Interventions version 6. Cochrane, Available from www. Cochrane Reviews take a systematic and comprehensive approach to identifying studies that meet the eligibility criteria for the review. This chapter outlines some general issues in searching for studies; describes the main sources of potential studies; and discusses how to plan the search process, design and carry out search strategies, manage references found during the search process, correctly document the search process and select studies from the search results.

This chapter aims to provide review authors with background information on all aspects of searching for studies so that they can better understand the search process. This chapter focuses on searching for randomized trials. Many of the search principles discussed, however, will also apply to other study designs.

Considerations for searching for non-randomized studies are discussed in Chapter 24 see also Chapter 19 when these are specifically for adverse effects. Other discussion of searching for specific types of evidence appears in chapters dedicated to these types of evidence, such as Chapter 17 on intervention complexity, Chapter 20 on economic evidence and Chapter 21 on qualitative evidence. An online Technical Supplement to this chapter provides more detail on searching methods. There is increasing evidence of the involvement of information specialists in systematic reviews Spencer and Eldredge and evidence to support the improvement in the quality of various aspects of the search process Rethlefsen et al , Meert et al , Metzendorf , Aamodt et al Cochrane Review authors should, therefore, contact their Cochrane Information Specialist at the earliest stage to find out what advice and support is available to them.

Authors conducting their own searches should seek advice from their Cochrane Information Specialist not only on which sources to search, but also with respect to the exact strategies to be run see Section 4. Cochrane Information Specialists are responsible for providing assistance to authors with searching for studies for inclusion in their reviews, and for keeping up to date with Cochrane methodological developments in information retrieval Cochrane Information Specialist Support Team a.

Most CRGs offer support to authors in study identification from the early planning stage to the final write-up of the review, and the support available may include some or all of the following:. Systematic reviews require a thorough, objective and reproducible search of a range of sources to identify as many eligible studies as possible within resource limits. This is a major factor distinguishing systematic reviews from traditional narrative reviews, which helps to minimize bias and achieve more reliable estimates of effects and uncertainties.

Searching beyond MEDLINE is important not only for ensuring that as many relevant studies as possible are identified, but also to minimize selection bias for those that are found. Relying exclusively on a MEDLINE search may retrieve a set of reports unrepresentative of all reports that would have been identified through a wider or more extensive search of several sources.

Time and budget restraints require the review team to balance the thoroughness of the search with efficiency in the use of time and funds. The best way of achieving this balance is to be aware of, and try to minimize, the biases such as publication bias and language bias that can result from restricting searches in different ways see Chapter 8 and Chapter 13 for further guidance on assessing these biases. Unlike for tasks such as study selection or data extraction, it is not considered necessary or even desirable for two people to conduct independent searches in parallel.

Systematic reviews have studies as the primary units of interest and analysis. A single study may have more than one report about it or record for it , and each of these reports or other records may contribute useful information for the review see Section 4.

For most of the sources listed in Section 4. There is, however, an increasing number of study-based sources, which link multiple records of the same study together, such as the Cochrane Register of Studies and the Specialized Registers of a number of CRGs see online Technical Supplement , and some other trials registers and regulatory and industry sources.

Processes and software to select and group publications by study are discussed in Section 4. All review authors and others involved in Cochrane should adhere to copyright legislation and the terms of database licensing agreements.

With respect to searching for studies, this refers in particular to adhering to the terms and conditions of use when searching databases and other sources and downloading records, as well as adhering to copyright legislation when obtaining copies of publications. The search for studies in a Cochrane Review should be as extensive as possible in order to reduce the risk of reporting bias and to identify as much relevant evidence as possible see MECIR Box 4.

Database selection should be guided by the review topic Suarez-Almazor et al , Stevinson and Lawlor , Lorenzetti et al When topics are specialized, cross-disciplinary, or involve emerging technologies Rice et al , additional databases may need to be identified and searched Wallace et al , Stevinson and Lawlor , Frandsen et al a.

Plan in advance the methods to be used for identifying studies. Design searches to capture as many studies as possible that meet the eligibility criteria, ensuring that relevant time periods and sources are covered and not restricted by language or publication status. Searches should be motivated directly by the eligibility criteria for the review, and it is important that all types of eligible studies are considered when planning the search. If searches are restricted by publication status or by language of publication, there is a possibility of publication bias, or language bias whereby the language of publication is selected in a way that depends on the findings of the study , or both.

Removing language restrictions in English language databases is not a good substitute for searching non-English language journals and databases. Searches for studies should be as extensive as possible in order to reduce the risk of publication bias and to identify as much relevant evidence as possible. Expertise may be required to avoid unnecessary duplication of effort. These databases are described in more detail in Sections 4.

A key advantage of such databases is that they can be searched electronically both for words in the title or abstract and by using the standardized indexing terms, or controlled vocabulary, assigned to each record see Section 4. They may be available through national provisions, site-wide licences at institutions such as universities or hospitals, through professional organizations as part of their membership packages or free-of-charge on the internet.

Some international initiatives provide free or low-cost online access to databases and full-text journals over the internet. Electronic Information for Libraries EIFL is a similar initiative based on library consortia to support affordable licensing of journals and other sources in more than 50 low-income and transition countries in central, eastern and south-east Europe, the former Soviet Union, Africa, the Middle East and South-east Asia.

The online Technical Supplement provides more detailed information about how to search these sources and other databases. The accompanying Appendix provides a list of general healthcare databases by region and healthcare databases by subject area.

Further evidence-based information about sources to search can be found on the SuRe Info portal , which is updated twice per year. MEDLINE as of January contains approximately 30 million references to journal articles in biomedicine and health from onwards.

When searching MEDLINE via service providers or interfaces other than Ovid or PubMed, we recommend verification of the exact coverage of the database in relation to PubMed, where no explicit information on this is readily available. Embase as of January contains over 35 million records from onwards, including records from more than currently published journals from approximately countries Elsevier Further details on the implications of this for searching are available in the online Technical Supplement.

Embase Classic provides access to almost two million records digitized from the Excerpta Medica print journals the original print indexes from which Embase was created from to Elsevier Embase now also includes pre-print articles from multiple sources Elsevier Embase is only available by subscription, either directly via Elsevier as Embase.

Note that Embase is searched regularly by Cochrane for reports of trials. The actual degree of reference overlap between MEDLINE and Embase varies widely according to the topic, but studies comparing searches of the two databases have generally concluded that a comprehensive search requires that both databases be searched Lefebvre et al , Bramer et al see MECIR Box 4. Conversely, two studies examined different samples of Cochrane Reviews and identified the databases from which the included studies of these reviews originated Halladay et al , Hartling et al Both studies, one across all healthcare areas Halladay et al and the other on child health Hartling et al , report a minimal extent to which the inclusion of studies not indexed in PubMed altered the meta-analyses.

PubMed coverage across systematic review topics has been further evaluated in a recent study based on a comprehensive sample of Cochrane Reviews. Hence, the current recommendation of searching multiple databases needs to be evaluated further, so as to confirm under which circumstances comprehensive searches of multiple databases are warranted.

CENTRAL, however, also includes citations to reports of randomized trials that are not indexed in MEDLINE, Embase or other bibliographic databases; citations published in many languages; and citations that are available only in conference proceedings or other sources that are difficult to access.

It also includes records from trials registers and trials results registers beyond ClinicalTrials. These additional records are, for the most part, identified by Cochrane Information Specialists, many of whom conduct comprehensive searches to populate CRG Specialized Registers, collecting records of trials eligible for Cochrane Reviews in their field.

In these cases, the search will be more precise, but an equivalent number of included studies will be identified with lower numbers of records to screen. Many review authors have full access free-of-charge at the point-of-use through national provisions and other similar arrangements, or as part of a paid subscription to the Cochrane Library. Many countries and regions produce bibliographic databases that focus on the literature produced in those regions and which often include journals and other literature not indexed elsewhere.

It is highly desirable that searches be conducted of appropriate national, regional and subject specific bibliographic databases see MECIR Box 4. Further details are provided in the online Technical Supplement. Citation indexes are bibliographic databases that record instances where a particular reference is cited, in addition to the standard bibliographic content.

Citation indexes can be used to identify studies that are similar to a study report of interest, as it is probable that other reports citing or cited by a study will contain similar or related content. Search appropriate national, regional and subject-specific bibliographic databases.

Databases relevant to the review topic should be covered e. Initiatives to provide access to ongoing studies and unpublished data constitute a fast-moving field Isojarvi et al It is important to identify ongoing studies, so that when a review is updated these can be assessed for possible inclusion. Awareness of the existence of a possibly relevant ongoing study and its expected completion date might affect not only decisions with respect to when to update a specific review, but also when to aim to complete a review.

Even when studies are completed, some are never published. Finding out about unpublished studies, and including their results in a systematic review when eligible and appropriate Cook et al , is important for minimizing bias. Several studies and other articles addressing issues around identifying unpublished studies have been published Easterbrook et al , Weber et al , Manheimer and Anderson , MacLean et al , Lee et al , Chan , Bero , Schroll et al , Chapman et al , Kreis et al , Scherer et al , Hwang et al , Lampert et al There is no easy and reliable single way to obtain information about studies that have been completed but never published.

There have, however, been several important initiatives resulting in better access to studies and their results from sources other than the main bibliographic databases and journals. These include trials registers and trials results registers see Section 4.

A recent study Halfpenny et al assessed the value and usability for systematic reviews and network meta-analyses of data from trials registers, CSRs and regulatory authorities, and concluded that data from these sources have the potential to influence systematic review results. A Cochrane Methodology Review examined studies assessing methods for obtaining unpublished data and concluded that those carrying out systematic reviews should continue to contact authors for missing data and that email contact was more successful than other methods Young and Hopewell An annotated bibliography of published studies addressing searching for unpublished studies and obtaining access to unpublished data is also available Arber et al One particular study focused on the contribution of unpublished studies, including dissertations, and studies in languages other than English, to the results of meta-analyses in reviews relevant to children Hartling et al They found that, in their sample, unpublished studies and studies in languages other than English rarely had any impact on the results and conclusions of the review.

Correspondence can be an important source of information about unpublished studies. It is highly desirable for authors of Cochrane Reviews of interventions to contact relevant individuals and organizations for information about unpublished or ongoing studies see MECIR Box 4. Letters of request for information can be used to identify completed but unpublished studies. One way of doing this is to send a comprehensive list of relevant articles along with the eligibility criteria for the review to the first author of reports of included studies, asking if they know of any additional studies ongoing or completed; published or unpublished that might be relevant.

This approach may be especially useful in areas where there are few trials or a limited number of active research groups. It may also be desirable to send the same letter to other experts and pharmaceutical companies or others with an interest in the area.

Some review teams set up websites for systematic review projects, listing the studies identified to date and inviting submission of information on studies not already listed. A recent study assessed the value of contacting trial authors and concluded that data supplied by authors modified the outcomes of some systematic reviews, but this was poorly reported in the reviews Meursinge Reynders et al Another case study of a Cochrane Methodology Review reported that making contact with clinical trials units and trial methodologists provided data for six of the 38 RCTs included in the review, which had not been identified through other search methods Brueton et al C31 : Searching by contacting relevant individuals and organizations Highly desirable.


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