Literature Searches and Literature Reviews: Literature Searches

Step 2. Choose the Resources to Search

Check several resources in your search, including: 

• Internet search engines such as Google and Google Scholar (see below). Search engines cover all subjects, retrieve many results and are free. However, these resources offer no guarantee that the information will remain unchanged or be available in the future. There is also no quality control of the information retrieved.
• Databases such as the Transportation Research International Documentation (TRID) Database (see below). Databases cover specific subject areas, can locate academic literature and provide a level of quality control to the content. Sometimes these resources are subscription-based, which limits access.
• Library catalogs such as WorldCat or library holdings for an individual library. Catalogs include monographs and theses, but typically exclude individual journal articles or conference papers, and rarely include the full text of materials. These resources are also free, but may be limited to specific audiences

See the Resources: Where to Search section of this guide for additional resources and information.

Note: Not all resources are publicly available or online. Librarians, especially those who specialize in the transportation field, are the best source of information about new databases or changes to existing databases. University, corporate or public librarians may have access to subscription databases and other sources that can provide resources free of charge or at a reduced cost.

Transportation Research International Documentation (TRID) Database

TRID provides access to more than 1.3 million transportation research records worldwide. It includes records from TRB’s Transportation Research Information Services Database, the OECD’s Joint Transport Research Centre’s International Transport Research Documentation (ITRD) Database and the Research in Progress (RiP) Database.

Searching TRID

• Each record in TRID is indexed using terms from the Transportation Research Thesaurus (TRT).
• The basic search mode looks for a keyword in all of the indexed TRID fields (title, abstract, notes, index terms, subject areas, authors, project managers or principal investigators, serial, corporate authors, publishers, and funding or performing organizations). Full-text search is not available.
  • A drop-down feature allows users to limit the search to the title, author name, agency, serial or conference title, or index term field. The basic search can also be limited to exclude research in progress or records without full-text links.
• The Advanced Search feature allows users to search one or more fields. Search results can be limited by subject area, language, publication type, source or date.

Viewing Results

• The top 25 results are displayed by publication date, beginning with the most recent resource.
  • Users can change the sort order from the drop-down menu.
• Search results provide a preview of the abstract and links to the full text if available from other websites.
• The full record can be accessed by clicking on the title.
• Users can mark records and then print, email, save or share them via social media.

Special Features

Links to recently published records, recently added records, advanced search features, search history, site help, rich site summary (RSS) feeds, recent records by mode, hot topics and recent TRB publications are availilable from the TRID homepage.

Google Scholar

Google Scholar, a subset of the Google internet search engine, provides access to scholarly literature across many disciplines. Sources include publications from academic publishers, professional societies, digital repositories and universities, and case law.

Searching Google Scholar

• The basic keyword search works much like the standard Google search. Google Scholar will search for keywords or phrases anywhere within the full text of the document.
• The Advanced Search feature allows users to search for exact phrases; specify that at least one of the words—or all words—from a set be in each result; exclude words from the search; and limit the search to the article title, a specific author or journal, or article date range.
• It does not have a thesaurus feature, and users cannot limit their search to a specific discipline.

Viewing Results

• Results are presented by relevance, which takes into account the full text of each document, its source, author, and how often and how recently it has been cited in other scholarly literature.
• Search results provide the title of the publication, author(s), source, year and links to the online version.
• Results include an excerpt of the article (with the search terms highlighted), the frequency of citation, related articles and a link to all available versions.
• The full text (or access to the publisher's site) cis available by clicking on a record’s link.
• Users can save records individually to a user library and create an alert to receive future documents matching the search criteria.

Special Features

The Scholar Settings feature allows searchers to choose the display language, choose the languages of the results, make results open in a new window, specify the citation manager for exporting links and set the Library Links feature to show library access links for up to five libraries.

Step 3. Choose Search Terms

To develop the initial search term list, use the words that describe the search topic developed in Step 1. Then:

• Identify synonyms, plurals and different word endings (e.g., climate and climatic).
• Consider technical, local and international terms, acronyms and abbreviations that are related to these words.
• Consider spelling variations (e.g., behavior and behaviour).
• Check the availability of a thesaurus, subject headings and index terms from the search resource and look up related terms.

The Transportation Research Thesaurus (see below) is a valuable tool for identifying search terms.

Note: When creating the list of search terms, keep in mind that acronyms can have different meanings for different audiences; words with diacritics (the marks above or below characters) may need to be searched separately (with and without the diacritic); and terminology may vary from country to country (e.g., truck vs. lorry).

Transportation Research Thesaurus (TRT)

The TRT is an online resource of standardized vocabulary that allows users to index, search and retrieve technical reports, research documents and other transportation information in the Transportation Research Information Services (TRIS) Database. Keywords provided in the TRT represent all modes and aspects of transportation—from construction, maintenance and materials to economic and social factors. With more than 9,500 terms, this free resource provides references to related words and concepts, improving information access to the transportation community.

The following resources provide information about the TRT:

• Transportation Research Thesaurus—A Powerful Tool for Finding Transportation Research Information. The major features and functions of the TRT are highlighted in this brochure.
• The Transportation Research Thesaurus (TRT): An Overview. The structure of the TRT, including how to search using a TRT term and how the TRT is updated with new terms, is summarized in this slide presentation.
• Video tutorials:
  • Why Use the Transportation Research Thesaurus: Your Multi-Tool for Finding and Organizing Transportation Information
  • Super Searching With the Transportation Research Thesaurus
• NCHRP Research Report 874: The Transportation Research Thesaurus: Capabilities and Enhancements. This report documents the results of a comprehensive assessment of TRT's capabilities and strategies for future development.
• NCHRP Report 450: Transportation Research Thesaurus and User's Guide. This report describes the development of the TRT.

Step 4. Compile the Search Strategy and Run the Search

Begin the search process at the help pages of the database, search engine or catalog that you are using to understand how searches work in that resource. Reading this information could save you a tremendous amount of time.

Then develop a search strategy that explores how terms are linked instead of simply adding any potential words into a single search field. Below are a few basic strategies for using keywords and search terms.

And, Or, Not (Boolean Operators)

Most databases and search engines support Boolean logic, allows users to broaden or restrict a search. Boolean functionality lets users:

• Retrieve specific search results by using the AND operator. 
• Broaden the search using the OR operator (used when similar terms refer to a concept).
• Exclude certain terms using the NOT operator.
• Use more than one Boolean operator by inserting parentheses to control the order of the search terms.

Advanced Search Tips
Most databases and library catalogs offer advanced search functions. Below are some of the most common advanced search options: 

• Exact phrase searching. Retrieves two or more words in the exact order they are typed.
• Truncation. Broadens the search by including all word endings. Common symbol: *,  $, % and ?.
• Wildcards. Used to substitute a symbol for one character. Common symbol for a wildcard: ?.
• Proximity searching. Allows users to specify how close together two words must appear. Common operation: NEAR.
• Controlled vocabulary. Used to help organize knowledge. Also referred to as subject areas, descriptors, thesaurus or index terms.
• Keywords. Retrieves items based on search terms chosen by the user.
• Field searching. Searches for keywords in all common fields of a database (e.g., title, author, abstract, index terms or subject headings).

Step 5. Review the Search Results

After running the initial search, review the results to determine if more searching is necessary and if the search strategy needs modification.

• Examine the relevant results to identify additional  keywords, index terms or subject headings.
• Consider whether the right sources and most relevant search terms (synonyms, truncations, Boolean operators) have been used.
• Note any recurring authors and conduct an author name search to potentially locate other relevant research.
• If the search produced limited results, consider broadening the focus of the topic or consulting sources other than online resources.

Looking Beyond Online Resources

• Do not immediately discount more general material in search results. Relevant information may be found as a component of a larger document and may not be indexed separately (e.g., a table within an article or a chapter within a book).
• Depending on the breadth of the search, factor in time to locate and review documents that exist only in print format.
• Contact transportation professionals or subject matter experts directly. They are often happy to share their expertise (or even a copy of their paper).

Knowing When to Stop

Knowing when to stop is subjective and is often based on time constraints. It's unrealistic to expect to find 100% of relevant research on a topic, regardless of the amount of time spent. An initial, focused effort of 3 to 5 hours of proper searching may yield 80% of all relevant citations that can reasonably be located using sound techniques in the proper sources. Spending another 10 to 20 hours may only yield 5% to 10% more relevant citations. Some citations may never be found due to indexing errors, timing and other variables. Consider whether continuing the search is producing new, relevant citations.

Step 6. Organize the Results

Record full bibliographic information (title, author, year of publication, journal title, volume number) and notes about the content (e.g., the database used to locate the resources, libraries that might house the resources) of each relevant citation. For extensive or ongoing searches, consider using a bibliographic management tool to organize the results and retrieved items. Bibliographic management software allows users to create a personal database of references to keep track of all citations—books, book chapters, journal articles, dissertations, recordings, webpages, letters, manuscripts and many other types of documents.

• Records can be entered manually or imported directly from many library catalogs and commercial databases.
• Once a record has been entered into the bibliographic management tool, users can:
  • Search for all records on a specific topic or by a certain author.
  • Quickly generate bibliographies of all or selected records.
  • Format references in a specific bibliographic style using word processing software.
  • Add relevant notes to citations and attach copies of full-text documents to citations.
• Three popular reference management products:
  • EndNote
  • Reference Manager
  • Refworks

RSS Subscriptions and Alerts

TRID and other databases allow users to share and organize search results using tools and social networking options directly from the search interface. They may also offer subscriptions to RSS feeds and alerts. RSS feeds automatically download updated information that meets search criteria to the user’s computer; alerts email updates of the latest relevant results to the researcher. These and other tools allow searchers to continue accessing the latest research on a topic long after they have completed the initial search.

Finding a Full-Text Document

Some databases provide links to the full text of a document or provide selective links. The full text may also be available through local or university library catalogs, e-journals with subscriptions, WorldCat, interlibrary loans and document delivery services, and direct contact with authors, publishers and sponsoring agencies.

Practical Tips From Transportation Librarians

• More is not always better. Strive for quality rather than quantity.
• As databases grow to include more and more records, it becomes increasingly important to know precise ways to search them in order to reduce the number of irrelevant results.
• Leverage the work already done by others. Review the references of a study or publication (sometimes called works cited or bibliography) for other potentially relevant sources.
• If you find something that is close to or exactly what you are looking for, look at the keywords and phrases from the abstract and reuse them in further searching.
• Zero in on the most unusual term in your search, sometimes searching with only that word, or that word plus one more.
• Transportation organizations change their names—sometimes often! Make sure that you use the latest version of the name for current research; use older versions when doing historic research.
• Remember to use British spelling variations of keywords and search terms. 
• Before you decide that nothing exists on your topic, ask the librarian who may find information that is buried on the internet or in books or magazines.
• A dead link may not be a dead end. Try the Wayback Machine.
• When you encounter a relevant resource online that isn’t free to access, ask your organization’s librarian. Chances are good the librarian can borrow or obtain the resource for free from another source.
• Sort citations in ways that will help those conducting the literature review. You may choose to sort them in order of importance, year, publisher or author.