Chapter 31: Searching the Literature

Although most of us have had some experience obtaining references for term papers or assignments, technology has forever changed how we locate information. Our expectations have changed in terms of how quickly we want the information, the volume of data available, and virtual accessibility. These expectations have required us to become better skilled at locating and using resources.

Clinicians may go to the literature to expand their knowledge or simply to stay on top of scientific advances as part of professional development and life-long learning. They may also gather information specifically for clinical decision making as part of the framework of evidence-based practice (see Chapter 1, Figure 1.2). Using the "best research evidence" available allows the practitioner to balance that information with clinical judgment and patient values to determine the most appropriate course of action. This process, of course, assumes that the clinician is able to locate the relevant research literature.

Researchers will review the literature in the development of a research question (see Chapter 7) and the interpretation of findings. The literature review is necessary to build the rationale for a study and to help make decisions on operational definitions and methods. Researchers who conduct systematic reviews require a comprehensive approach to searching for primary sources of data on a given topic (see Chapter 16).

In other words, we all need to develop skills in searching the literature so that we can find what we want when we need it. The purpose of this chapter is to describe strategies for successful literature searches.

There are many efficient strategies for locating research references. The library, of course, is where this process begins, whether this is an actual building or a virtual connection. Clinicians may have access to a departmental or institutional library, but medical and university libraries will usually be more complete. Most libraries today have online catalogues that allow searching by author, title or subject. Articles, books or theses that are not available at a local facility can often be obtained electronically or through interlibrary loan. Many facilities also provide access to full text downloads of articles, sometimes for a fee. Technology has greatly enhanced everyone's access to literature searches through Internet resources.

Many specialized library resources are available to assist the researcher in locating references on specific topics. Search engines are information retrieval systems that search the Internet and electronic databases for websites, files or documents based on keywords or phrases. Meta-search engines query several other search engines and/or databases simultaneously, allowing the user access to broader resources by entering search criteria only one time.

Some search engines specialize in health and medical topics (see Table 31.1). The National Center for Biotechnology Information (NCBI), which is part of the National Library of Medicine, offers an integrated, text-based search and retrieval system called PubMed. Scirus focuses on scientific, scholarly, technical and medical information. Google Scholar searches scholarly literature in journals, theses, books, as well as material from professional societies, preprint repositories, universities and other scholarly organizations. The OVID search engine provides access to over 300 databases in medicine and health.

Many professional associations offer access to search engines with specific relevance to their particular field. The Open Door portal is available to members of the American Physical Therapy Association (APTA), providing access to full text clinical and academic journals, conference proceedings and dissertations related to rehabilitation. The American Occupational Therapy Association (AOTA) sponsors OT Search, available by subscription. The American Speech-Language-Hearing Association (ASHA) supports The Dome, a search engine for communication sciences disorders professionals, also available through subscription.

Databases are organized systems that allow search access for specific content or information. Bibliographic databases contain lists of citations of published and unpublished references. A computer search allows the researcher to view the full citation and abstract of a document, to determine if it is relevant. The citation can then be printed or downloaded for future reference. Citations may be displayed as the summary bibliographic reference, reference with abstract, or full text of the article.

Several databases are available for health science literature, providing listings of articles, books, theses and dissertations, and conference proceedings (see Table 31.1). Many databases can be accessed free through the Internet, while others require a subscription through an online service or CD-ROM. The list that follows is by no means exhaustive.

• The Clinical Trials Registry provides a regularly updated information databank about federally and privately funded clinical research. The database provides information about a trial's purpose, eligible subjects and locations. The site is geared toward non-health professionals who want to learn more about ongoing human studies research.

• The Cumulative Index to Nursing and Allied Health Literature (CINAHL) provides access to citations from all nursing journals as well as primary journals in more than a dozen other health disciplines, including occupational and physical therapy, speech language pathology, cardiopulmonary technology, respiratory therapy, nutrition and social services.

• The Educational Resources Information Center (ERIC) hosts a database of education-related resources, sponsored by the Institute of Education Sciences of the U.S. Department of Education. A newly designed digital library allows free access, including full text availability of many journal and nonjournal materials.

• EMBASE is the Excerpta Medica biomedical database, which includes over 7,000 journals covering medicine, pharmacology and drug research, and health policy and management. EMBASE includes MEDLINE references from 1966, and additional references from 1974 on.

• The Index to Scientific & Technical Proceedings provides information on papers delivered at major conferences. It includes bibliographic information and abstracts from proceedings published in journals and books. This database covers topics included in the Science Citation Index. An Index to Social Science and Humanities Proceedings is also available.

• Latin American and Caribbean Health Sciences Information System (LILACS) is a cooperative database published since 1982 that registers health-related literature published by authors in Latin American countries. The database includes journal articles, conference proceedings, scientific-technical reports, theses, books and book chapters. LILACS indexes 670 journals published in the region, with abstracts in English, Portuguese or Spanish; only 41 of these journals are also included in MEDLINE or EMBASE. Therefore, a literature search can be substantially enhanced by including this database.¹ Subscription is required. LILACS is part of the WorldCAT network.

• MEDLINE is the most commonly used database in health-related sciences, indexing almost 5,000 journal titles in biomedical sciences. It is the electronic platform for the formerly published Index Medicus. Full references are available back to 1966, although earlier citations from 1950-1965 can be obtained through Old MEDLINE. MEDLINE is part of MEDLARS (MEDical Literature Analysis and Retrieval System), the computerized system of databases offered through the National Library of Medicine (NLM). This database can be accessed free through the PubMed website. PubMed also allows the search to be limited to subsets of databases that focus on AIDS, bioethics, cancer, complementary medicine, and other topics. PubMed Central (PMC) provides free full-text access to articles in a growing number of journals.

• National Rehabilitation Information Center (NARIC) houses the REHABDATA database, which contains approximately 69,000 abstracts of books, reports, articles, and audiovisual materials relating to disability and rehabilitation research, dating back to 1956. NARIC also includes the NIDRR Project Database with information on every project funded by NIDRR since 1993. Each record includes institutional and contact information, funding data, and an abstract of project goals and activities.

• OTseeker was developed by occupational therapists at the University of Queensland and the University of Western Sydney. It contains abstracts of systematic reviews and clinical trials related to occupational therapy, and includes ratings based on the PEDro scale. In 2006, more than 4,300 articles had been reviewed.

• PEDro is the Physiotherapy Evidence Database, which is an initiative of the Centre of Evidence-Based Physiotherapy out of the University of Sydney. This database includes citations and abstracts of systematic reviews, clinical trials and clinical practice guidelines related to physical therapy. All clinical trials are rated for quality using the PEDro Scale, a standardized rating scale for assessing methodologic quality (see Chapter 16).

• ProQuest Dissertations and Theses (PQDT) is a comprehensive collection of graduate scholarly works from universities, cataloguing more than 2 million dissertations and theses from around the world. Citations date from the first accepted U.S. dissertation in 1861, although abstracts are available only since 1980. Most are available in full text in microfilm or hard copy, and more recent titles may be obtained in electronic formats, depending on subscriptions. Proquest includes the former University Microfilms (UMI) collections through Dissertation Abstracts.

• PsycINFO is an abstract database sponsored by the American Pyschological Association. It covers topics in psychology, behavioral sciences and mental health, including over 2,000 journal titles, books and book chapters. Historic records are available dating back to 1802. Abstracts of dissertation titles are listed starting from 1995.

• Public Library of Science (PLoS) is a nonprofit organization of scientists and physicians who are committed to making medical research literature a public resource. Open-access journals are published in biology, medicine, computational biology, genetics, pathogens, and clinical trials. The inaugural journal was published in 2004.

• SCOPUS is available to research and academic institutions. It covers over 15,000 titles including journals, conference proceedings, books and trade publications. It includes scientific, technical, medical, and social science literature, with sources from the United States, Europe, Latin America and Asia. Scopus includes all of MEDLINE and more than 500 open access journals.

• SportDiscus is a database supported by the Canadian Sports Information Resource Centre (SIRC). It covers sports, exercise physiology, sports medicine, physical fitness, and biomechanics. It indexes over 2,000 journals as well as books, theses, and nonprint resources.

• Web of Knowledge is an integrated database that allows searching for citations, conference proceedings, websites and books in a multidiscliplinary platform. Its records cover scholarly information in the sciences, social sciences, and arts and humanities going back more than 100 years.

• WorldCat is a library network that allows searching collections of more than 57,000 libraries around the world. Books, theses and dissertations, as well as special collections are catalogued through this service. WorldCat is part of the Online Computer Library Center (OCLC), which provides various services to an international consortium of libraries. Most academic libraries are members of OCLC. The database identifies libraries that hold the items of interest, and will usually link directly to that library's website. The availability of the item will depend on that library's policies.

The most important part of a literature search is deciding what terms to enter as keywords. All electronic databases will utilize some form of algorithm to generate a list of citations based on the keywords you choose. Citations will generally be brought up if the keyword is present in the title or abstract of the article. Journal articles may also have lists of keywords assigned to them by the author that are then used to index the document. Because so many words have synonyms or alternative terminology, this process can be daunting at times. It is useful to begin by brainstorming the various words that may be useful in finding articles related to your topic. Consider the pathology or diagnosis, the population, the specific treatments or measurement tools, and the outcomes of interest.

Suppose we were interested in studying physical endurance in patients who have experienced a stroke, with specific interest in the effectiveness of exercise programs. We would want to learn about the previous success of exercise programs for this population, and how endurance has been measured. Some possible keywords might include:

Once we choose our keywords, we must consider how to combine them to generate citations that will relate to our question.

Boolean Logic

When performing a search, it is important to fine tune the choice of key words or reference terms to narrow the search. Most databases and search engines use a system called Boolean logic, named for George Boole, an English mathematician who invented it in the mid 1800s. Boolean logic utilizes three primary operators: AND, NOT and OR.^∗

We will illustrate the use of these operators to develop a search strategy for our question related to endurance after stroke (see Figure 31.1).

AND narrows the search, requiring that all requested terms to be present. We can begin by finding references that include both of our main terms:

stroke AND exercise

This search yields 6,423 citations.^†

OR broadens the search, usually when you want to include synonyms of the main search term. We may want to expand the search so that it includes studies that use the terms stroke or cerebrovascular accident:

(stroke OR "cerebrovascular accident") AND exercise

This has broadened the search to 6,448 citations. The term in quotes is read as a fixed string, not individual words. The terms that appear in parentheses are searched first and then paired with the second command. If more than one set of parentheses is present, the search sequence goes from left to right.

NOT excludes concepts from the search. Suppose, for example, we want to eliminate studies that exclude patients with diabetes:

stroke AND exercise NOT diabetes

This brings the search down to 6,084 citations.

We may finally want to limit our search to outcomes related to endurance:

stroke AND exercise AND endurance

The search is now limited to 439 references. Perhaps we can refine it a little more by making our terms more specific. For example, let's specify "physical endurance" to clarify the type of endurance we want to include:

stroke AND exercise AND "physical endurance"

Now our search is down to 278 citations. This is still too large a number of references, however. Let's examine how we can move through this process more effectively.

MeSH Headings

The National Library of Medicine has developed a sophisticated system of Medical Subject Headings (MeSH) that have been adopted by most databases that cite medical and health literature. Because authors will use different terminology for the same concepts, MeSH terms provide a consistent search vocabulary. MeSH consists of sets of terms in a hierarchical structure or "tree" that permits searching at various levels of specificity.² The tree starts with broad categories such as "Anatomy," "Disease" or "Analytical, Diagnostic and Therapeutic Techniques and Equipment." More specific terms, close to 23,000 descriptors, are provided at various levels of the tree. Each heading can be further detailed in a list of subheadings. Table 31.2 shows a partial list of terms to illustrate this hierarchy.

Additional options include exploding the search, which means that the search will retrieve results using the selected term and all of the more specific terms in the tree.

Focusing or restricting the search means that it will be limited to documents where the subject heading is the major point of the article.

OVID provides a "Map Term to Subject" feature that shows MeSH headings for entered keywords. PubMed provides a MeSH database that will serve the same purpose. These programs provide a useful tutorial for those who are not familiar with this process.

For our current example, the term stroke is not a MeSH heading, but "cerebrovascular accident" is. There are several terms that relate to exercise, including "exercise therapy," "exercise test," "exercise movement techniques," and "exercise tolerance." The definition of "exercise therapy" is closest to our intention: "Motion of the body or its parts to relieve symptoms or to improve function, leading to PHYSICAL FITNESS, but not PHYSICAL EDUCATION AND TRAINING." MeSH terms related to endurance include "physical endurance," "sports," and "muscle fatigue." The definition for "physical endurance" is most appropriate for our purpose: The time span between the beginning of physical activity by an individual and the termination because of exhaustion.

In our search we use a slash to define our terms as MeSH headings. By combining the three terms:

cerebrovascular accident/AND exercise therapy/AND physical endurance/

We now obtain 9 references, substantially different from the 278 we had before!

A comprehensive search strategy for this topic is illustrated in Box 31.1, showing the scope of this process. This search was used to generate references for a systematic review.³

Limiting the Search

Search strategies can be refined by setting specific limits or filters, including language, dates of publication, or studies that are provided as full text. Searches can be restricted to studies of humans or animals, specific age groups or gender, or types of studies such as case studies, randomized trials, or systematic reviews. A specific author or journal can be searched. When a search yields large numbers of potential references, such limits will often provide a useful way to hone in on articles that are available and contemporary.

PubMed has developed predesigned strategies to target a search for studies on therapy (interventions), diagnosis, etiology or prognosis. These search strategies can be accessed under Clinical Queries.

Truncation and Wildcards

Most databases will recognize truncation and wildcard symbols that can be used to allow a broad interpretation of a search term, finding plurals, spelling variations and alternate forms of words. Each database specifies the symbol to be used, such as a question mark (?), an asterisk (∗) or a dollar sign ($). For example, entering the term "sensitiv$" in PubMed will retrieve articles that include "sensitive" or "sensitivity." The term "wom$n" will find "woman" or "women."

Broadening the Search

Related References

Several databases offer an option to search "related articles" or "similar pages," once a relevant citation has been found. By using this function, one relevant article becomes the key to a list of useful references. These references will usually be shown in order of their relevance to the original record.

Science Citation Index

Science Citation Index is a reference system that allows researchers to search based on citations (see Table 31.1), by tracking literature forward or backward, and across disciplinary boundaries. The index will identify articles, books or other sources that have cited a particular reference or author. This index is available through online subscription or DVD, usually through a library.

Other Articles' Reference Lists

The reference list at the end of published articles will provide a handy inventory of sources that were used by other authors in their review of literature. The obvious disadvantage of this approach is that you will only identify references that are older than the original article. But it is often a good place to start, and to distinguish classic references that are used repeatedly.

Sensitivity and Specificity of Literature Searches

Electronic searching can be considered a blessing or a curse by the researcher. It is certainly more efficient than the old-fashioned time-consuming hand searching through volumes of the Index Medicus, although at the same time it can generate substantial lists of citations that may not be relevant to the project. The incentive for developing competence in searching is the ability to avoid retrieving too many citations or long lists of irrelevant ones.

In this aspect of searching, the terms "sensitivity" and "specificity" have been applied, with meanings analogous to their use with diagnostic tests (see Chapter 27).⁴ Sensitivity, also called recall, is the proportion of retrieved citations that are relevant, or the likelihood of finding relevant references. A sensitive search might include some irrelevant studies but is more likely to be comprehensive. Specificity, also called precision, is the proportion of citations that are relevant that the search is able to retrieve, or the likelihood of excluding irrelevant references. A specific search might exclude some relevant studies but is more likely to include the very relevant ones.

When a search results in too many citations, you need to improve specificity by narrowing the question, using more specific search terms, combining search terms, or setting limits. Using the "focus" option can also restrict the search. When the number of references is too small, the search needs to improve its sensitivity by broadening the question, finding better search terms, using truncation or wildcards or trying varied combinations of terms. Using the "explode" option can improve the breadth of the search. In PubMed, under Clinical Queries, choosing a "sensitive" search will explode the search to include a broad range of articles, some that may be less relevant. Choosing a "specific" search will focus the search on the most relevant articles, but will eliminate some that have less obvious relevance. A useful guide to terms that can be used to increase sensitivity or specificity has been developed for use with MEDLINE.⁵

In Chapter 1 we described the process of generating a question related to a patient case (see Box 1.1) to provide an evidence base for clinical decision making. When we search the literature as part of evidence-based practice, we are not "studying" a topic, as suggested in the previous section. Instead, for this purpose we are asking a question that is spurred by a particular patient issue, and our literature search is intended to find references that will help us make a good clinical decision for management of the patient.

Using PICO

The acronym PICO has been used to emphasize the importance of specifying the Population (or Patient characteristics), Interventions, Comparisons and Outcomes that are relevant to the clinical question.⁶ In going through this process, it is useful to generate a thorough list of terms that can then be used as keywords in a literature search. For example, consider the following case scenario:

The therapist wants to gather specific information that the patient will be able to use in setting his own goals for exercise. She asks the question, "Will exercise improve endurance in a patient following stroke?

We might consider the following elements important for answering this clinical question:

These terms would become useful keywords for a search. Terms in quotes are intended to be searched as a string.

Searching for Systematic Reviews

Identifying systematic reviews in a search strategy can be challenging because the phrases "systematic review" or "meta-analysis" will not always appear in the title or abstract of an article. Many search engines and databases have established specific search strategies to find these types of studies. For instance, in PubMed reviews on a given topic can be obtained by checking "systematic reviews" under Subsets on the "Limits" tab. OVID provides limits for "Reviews" or "EBM Reviews." CINAHL will allow for retrieval of systematic reviews under publication type. Systematic reviews are considered a higher level of evidence than single primary studies (see Chapter 16) because they provide a critical analysis of available literature. Clinicians will typically find systematic reviews to be a good place to start when asking a clinical question for evidence-based practice decisions.

Reviews and Abstracts for Evidence-Based Practice

Several specialized databases index reviews of published literature. Evidence Based Medicine Reviews (EBMR) is a collection of databases available through the Cochrane Library and OVID Technologies, containing the following resources (see Table 31.1):

• ACP Journal Club is a collection of structured abstracts supported by the American College of Physicians. The abstracts describe the objectives, methods, results and evidence-based conclusions of original articles and systematic reviews published in over 100 clinical journals. The abstracts are written by clinicians with specific expertise to summarize the literature and provide brief commentaries on the context, methods and clinical applications of the findings. Articles are graded for clinical relevance using star ratings from 1 (least informative) to 7 (highly relevant).

• The Cochrane Database of Systematic Reviews is part of The Cochrane Library. This database indexes systematic reviews of healthcare interventions. Summaries of reviews can be accessed free online, but full text requires a subscription.

• The Cochrane Central Register of Controlled Trials (CCTR) is an index of randomized clinical trials that have been determined to be of sufficient quality for use in systematic reviews. CCTR can be accessed through the Cochrane Library.

• The Database of Abstracts of Reviews of Effects (DARE) is a full text database containing critical assessments of systematic reviews from a variety of medical journals. DARE records cover topics such as diagnosis, prevention, rehabilitation, screening, and treatment.

Other review databases are also useful for research and clinical decision making (see Table 31.1):

• BMJ Clinical Evidence is a journal of systematic reviews related to a broad range of clinical specialties. This publication continually updates already published systematic reviews by including recently published clinical trials as well as observational studies that might not have been cited in the original paper. The reviews are intended to summarize the current state of knowledge or uncertainty about the prevention and treatment of clinical conditions, and also strives to show when good evidence is not available.

• Evidence-based Practice Centers (EPC) Reports are developed on topics relevant to clinical, social science/behavioral, economic, and other health care organization and delivery issues—specifically those that are common, expensive, and/or significant for the Medicare and Medicaid populations. In 1997 the Agency for Health Care Policy and Research (AHCPR), now known as the Agency for Healthcare Research and Quality (AHRQ), launched its initiative to promote evidence-based practice in everyday care through establishment of 12 Evidence-based Practice Centers (EPCs).

• Hooked on Evidence is available to members of the American Physical Therapy Association. This database includes extractions from clinical trials, case-control and cohort studies, case reports and single-subject design studies related to physical therapy interventions. Over 3,000 extractions were available as of 2006.

The volume of literature that is available in health care can be truly overwhelming, as one strives to stay up to date in an environment that is increasingly focused on evidence. Many of us subscribe to a variety of journals that concentrate on our areas of clinical interest, and just keeping up with that reading can be a daunting task. Scanning the table of contents of particular journals can be a useful habit for finding articles of interest.

A convenient strategy to maintain a current library is to subscribe to email alert services, which provide regular automatic updates based on a search profile that you create. These services are usually free. They deliver current citations (with abstracts) or the table of contents of selected journals directly to your email. Services such as OVID, PubMed (through My NCBI), and ProQuest offer email alerts.⁷ Many journals, including Physical Therapy, Archives of Physical Medicine and Rehabilitation, New England Journal of Medicine, and the Journal of Speech, Language and Hearing Research, allow individuals to register to receive free alerts for their table of contents each month. Some journals will also send out alerts for early release of certain articles. In this day of electronic communication and information overload, it seems prudent to let technology help us sort through the many resources that are available to us!