Introduction

The Cochrane Oral Health Group is part of The Cochrane Collaboration and comprises an international network of researchers involved in producing and disseminating systematic reviews of randomized controlled clinical trials in the field of oral health. Searching for trials information to include in systematic reviews is a complex process; in order to avoid bias in the results of the review as many relevant trials as possible must be found. There are many sources that can be searched, including MEDLINE and EMBASE (Excerpta Medica Database). However, these databases are growing month by month and advanced searching techniques are required to ensure that all relevant studies are found, but not at the cost of being overloaded by too many citations.

Searching electronic databases for systematic reviews requires a balance between sensitivity (number of relevant articles found as a proportion of all the relevant articles) and precision (the number of relevant articles found as a proportion of all articles). Searches for Cochrane systematic reviews attempt to aim for maximum sensitivity so that no relevant articles are missed. This chapter will cover which databases to search and how to construct a sensitive search strategy.

Where to Search: Choosing Databases

No one single resource covers all the information that is needed for a systematic review. A range of databases should be searched in order to make sure that all eligible trials are found and included. A search will normally cover the more mainstream medical databases, MEDLINE and EMBASE, and trials and systematic review information within The Cochrane Library as a minimum. Non-English language literature, gray literature, and trials registers are further sources of reports of clinical trials.

MEDLINE

MEDLINE is a resource from the United States, based at the National Library of Medicine. Records date back to 1949, and 5000 journals have been added to the resource. It currently contains over 16 million citations in 37 languages (National Library of Medicine 2008) and has a well-deserved reputation as the most comprehensive medical science database (Collins 2007). MEDLINE is available through several database providers, including Ovid and EBSCO, via a subscription. MEDLINE is also available for free online via the PubMed service at http://www.ncbi.nlm.nih.gov/sites/entrez.

EMBASE

EMBASE, the European equivalent of MEDLINE, is based in the Netherlands and produced by the publisher Elsevier. It has coverage of over 7000 journals since 1974 and has 20 million citations (Elsvier 2009). It has a particular focus on pharmacological sciences and also provides access to non-English language references. Like MEDLINE, it is available as a premium subscription service via several database providers includ­ing Ovid. The free online version of EMBASE is available at http://www.embase.com/home.

The Cochrane Library

The Cochrane Library, from Wiley Interscience, is produced by The Cochrane Collaboration. It contains several useful resources, which can be searched simultaneously. The Cochrane Database of Systematic Reviews contains all the published Cochrane reviews and protocols; at the time of writing there were nearly 6000 records covering all the subject areas of the Cochrane Review Groups (Wiley Interscience 2009). The Cochrane Central Register of Controlled Clinical Trials (CENTRAL) comprises the trials registers collated and maintained by the Cochrane Review Groups and currently contains over 600,000 clinical trials (Wiley Interscience 2009). In addition, The Cochrane Library includes access to the Database of Abstracts and Reviews (DARE) produced by the Centre for Reviews and Dissemination at the University of York, a specialized methodology register and collection of technology assessments and economic evaluations. Access to The Cochrane Library varies from country to country, but it is free to residents of many countries, including the following: the UK, Australia, Canada, Denmark, Finland, India, Ireland, some Latin American and Caribbean countries, New Zealand, Norway, Poland, Spain, and Sweden. Access may also be available in other countries through individual or higher education library subscriptions.

The Cochrane Library can be accessed through The Cochrane Collaboration’s website at http://www.cochrane.org.

Non-English Language Literature

MEDLINE, EMBASE and CENTRAL within The Cochrane Library all provide access to non-English language citations of clinical trials, but there are alternative sources of information. One of the largest non-English language databases is the Latin American and Caribbean Health Sciences Literature Resource (LILACS), which provides access to references from journals published in South and Central America. It can be searched in English, Spanish, or Portuguese. Access in the UK can be gained through the Virtual Health Library (http://www.bireme.br). There are country-specific databases with some limited trials information, such as KoreaMED (http://www.koreamed.org). Other non-English language sources include the Chinese biomedical databases provided through Infobank (http://www.infobank.cn) and the various databases provided through the World Health Organization (WHO). These include resources for the eastern Mediterranean (http://www.emro.who.int/his/vhsl), Southeast Asia (http://library.searo.who.int/), and Africa (http://indexmedicus.afro.who.int).

Trials Registers

Information about clinical trials, both ongoing and completed, can be found on trials registers. Cochrane Review Groups all maintain a specialized register of trials in their subject area. The Cochrane Oral Health Group’s register currently contains approximately 25,000 references to published clinical trials information. Access to the Cochrane trials registers is normally arranged through the Review Group’s Trials Search Coordinator (more information can be found on www.cochrane.org). Information on ongoing trials in the UK can be found on the Meta-Register of Controlled Clinical Trials (http://www.controlled-trials.com/mrct/), a resource that is free to search and that gives details regarding the study design, outcomes, and contact information. The U.S. National Institutes of Health provide free access to http://clinicaltrials.gov, a database containing over 80,000 trials records (NIH 2009). Its coverage aims to be global, but there is an inevitable concentration on trials from the United States. The information provided includes the trial’s purpose, participants, and contact information. WHO provides a gateway to several trials registers at http://www.who.int/trialsearch. The registers covered include the Australia and New Zealand Clinical Trials Registry, the Chinese Clinical Trial Register, the Indian Clinical Trials Registry, the German Clinical Trials Register, the Iranian Registry of Clinical Trials, and the Netherlands Clinical Trials Register (WHO 2009). In addition to these trials registers, pharmaceutical companies also maintain registers of the clinical trials they have conducted. Many of these have been brought together by the International Federation of Pharmaceutical Manufacturers and Associations and published online via a gateway that is free to search at http://clinicaltrials.ifpma.org. This provides access to information such as trial sponsors, the condition and intervention being investigated, start and completion dates, and inclusion and exclusion criteria.

Gray Literature, Dissertations, and Conference Proceedings

Gray literature is that which is not formally published in books or journals. Along with dissertations and conference proceedings, it can be a useful source of trials information. OpenSIGLE (http://opensigle.inist.fr/) is the System for Information on Grey Literature in Europe and is a database of references relating to reports, dissertations, and conference papers. Although it ceased to be fully updated in 2005, it is still worth searching for older material. Access is free of charge. Conference proceedings can be found via a number of resources, including ZETOC (http://zetoc.mimas.ac.uk), the Web of Knowledge (http://isiwebofknowledge.com), and British Library Direct Plus (http://tinyurl.com/a27pc8). All of these are normally free to staff and students within UK higher education institutions, but they are also available via a subscription to those outside UK universities. Selected dissertation abstracts are also available online. Ethos (http://ethos.bl.uk) is a service provided by the British Library and has 250,000 records of abstracts of dissertations from UK universities. Database provider ProQuest also provides a dissertation and thesis service, although this is available by subscription only.

Choosing the Right Platform

Many of the resources listed above are available via several different service providers: platforms that offer access to these electronic databases include Ovid, EBSCO, PubMed, EMBASE.com, and SilverPlatter. Some of these require a subscription to access, some (PubMed, EMBASE.com) are free of charge. The subscription services are normally superior in that they allow more sophisticated and advanced searching and sometimes provide links to the full text of the citation. In most cases where there is access to both, the subscription services should be used in preference to the free versions. Most university and medical libraries subscribe to at least one of the subscription services, and advice should be sought from a subject specialist or librarian as to which are available and how to access them. Search syntax and subject headings vary from platform to platform, so it is important to know how the database is being accessed so that the search can be tailored appropriately. A search designed for MEDLINE via Ovid will not work in MEDLINE via PubMed. All of the mainstream medical databases provide help on their websites to assist in a correct and structured search strategy.

How to Search: Constructing a Search Strategy

Electronic Records

Most of the electronic databases mentioned above provide access to citations from journals, not the full text of the article. Some also contain access to citations from books, conference proceedings, and dissertations. Electronic records normally contain basic information about an article such as authors, title, journal, volume and issue, page numbers, language, and year of publication. In most cases, more detailed information can also be found, such as an abstract and contact details for the authors, although some older articles may have been added without abstracts. Many of the databases above also index all the journal articles with keywords and controlled vocabulary to help in searching.

Controlled Vocabulary

Most of the mainstream medical literature databases can be searched using a mixture of controlled vocabulary and free text. Controlled vocabulary is a list of words and phrases used to “tag” information in electronic databases in order to group similar articles together. The most famous example in this context is MEDLINE’s Medical Subject Headings (MeSH). MeSH terms are arranged in a hierarchy, or tree. Broader concepts come near the top of the tree and more specific terms lower down.

These subject headings are then assigned to the articles in MEDLINE by experienced indexers at the National Library of Medicine (NLM) in the United States. MeSH can be found for a topic by visiting the NLM’s MeSH browser at http://www.nlm.nih.gov/mesh/MBrowser.html. Typing in a keyword will not only give the MeSH term for that topic, but it will also show where the term appears in the MeSH tree. The MeSH term can be used in MEDLINE to search for any records that have been indexed with it.

MeSH can also be “exploded” to include all of the terms that are included in that subject heading on the tree. For example, exploding the term “Orthodontic Appliances, Removable” (Table 3.1), would also search the terms “Activator Appliances” and “Extraoral Traction Appliances.” However, you can also focus your search by not exploding the term. An unexploded search for “Orthodontic Appliances, Removable” would only retrieve the records indexed with that term and not the records indexed with “Activator Appliances” and “Extraoral Traction Appliances.”

Table 3.1 Example of a MeSH tree on orthodontic appliances.

Source: National Library of Medicine, Medical Subject Headings.