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Searching the Scientific Literature


This method uses "subject" keywords to search literature databases and catalogs. To effectively conduct a subject search you should first develop a subject search strategy and then think about how you will enter your search in a literature database or catalog as shown in the following example:

subject search form

Develop a Subject Search Strategy

Start by constructing a search strategy using the following four steps. 

STEP 1 -- Summarize your research topic in one or two sentences.

I am interested in finding research on the effect of sedimentation on the survival of salmon eggs.

STEP 2 -- Identify the unique ideas or concepts associated with your research topic.

A research topic typically has two or three unique concepts. In the sciences these concepts commonly fall into one of these categories: subject; taxonomic; geographic; time; habitat; life stage, population or age group; organ system; chemical substance; genetic sequence; disease; or methodology, technique or test.

Search Topic Concepts 

STEP 3 -- Brainstorm appropriate keywords for each concept.

Each concept usually can be described using several specific keywords. These keywords can be developed in several ways - your personal knowledge of the topic, suggestions of others, or background reading that you do in secondary sources. In making a list of keywords consider the following:

  • "Controlled Keywords"  Many literature databases and catalogs use "controlled" keywords that come from associated thesauri or lists of keywords and which are assigned by indexers to every record in the database. These "controlled" keywords bring together similar ideas under one standardized word or phrase that may be described in the discipline literature by several different keywords. See this reference as an example of the "controlled" keywords used in an literature database. In a database record they may be called "descriptors", "subject headings", etc.

  • Hierarchical Relationships  In developing keyword lists consider possible hierarchical relationships within a particular concept. For example, with a taxonomic concept are you only interested in locating research on a particular species or is a broader taxonomic classification also of interest?
marine biology

marine flora

marine algae

red algae (Rhodophyta)

coralline algae (Coralinales)


Sporolithon ptychoides

  • Scientific Nomenclature  For taxonomic concepts use both common and scientific names of organisms. Using both will normally increase the number of citations retrieved.
salmon or oncorhynchus
  • Word Truncation Examine each keyword to see if it can be beneficially truncated to retrieve variant forms of the word. This is especially true for single and plural variants of a word. You can use wildcard symbols (e.g., *, #, ?, +) available in a database to truncate words back to a base root. To find the correct wild card symbols to use in a database check its help section. Examples:

sediment* retrieves sediment, sediments, sedimentation 
silt* retrieves silt, silts, silting, silted, siltation

STEP 4 -- Establish the relationship between each keyword and concept.

Use Boolean operators -- AND, OR, and NOT -- and nesting to connect together every keyword and concept in your search statement. AND connects each concept and OR connects the synonymous keywords under each concept. Keywords connected by ORs need to be entered in the same search box or enclosed within parentheses ( ) when searching a database. See the sample search above as well as the following example:

(sediment* or silt*) and (hatch* or survival) and (salmon* or oncorhynchus) and (egg* or embryo* or redd*)

Strategize How to Enter Your Search in Literature Databases and Catalogs

Sometimes it is useful to conduct a preliminary search before beginning your main search. This search serves two purposes: 1) it is a means to get a basic sense of the literature on your topic; and 2) it serves as a source of keywords for each of your concepts and it will help you assess whether searching on specific keywords is likely to yield relevant results.

Concepts within a topic are often a mixture of specific and broad ideas. A useful approach is to identify the most specific concept and search that one first. If this initial search retrieves only a few references, just browse through them and identify the ones relevant to your topic. If the search retrieves many references, add another concept using the "and" connector to decrease your results.

Use "controlled" keywords as described above. If you do not know what "controlled" keywords to use, conduct an initial search using the keyword(s) you have. In reviewing the search results look for "controlled" keywords, often called descriptors or subject headings, which commonly appear as part of each citation (see example). Re-enter your search adding these "controlled" keywords to your existing keywords.

Use a "building block" approach to searching if the database you are searching allows for it. Enter each of your concepts individually by ORing together the list of synonymous keywords you have created, e.g., prevent* or avoid* or deterr*. After each of your concepts has been entered use the database "Search History" feature, if available, to AND together each of the concepts. Employing this approach allows you to:

  • add new keywords you have identified to an existing concept, e.g., concept 1 or avert* (new keyword)
  • try different concept combinations using the AND connector, e.g., 

concept 1 and concept 2 and concept 3
concept 1 and concept 3
concept 3 and concept 2

Searching is a dynamic process. As you proceed in your literature search, and as your personal knowledge increases, your list of keywords is likely to grow and/or be refined.

Boolean Operators (AND, OR, NOT)

Using the Boolean operators (AND, OR, NOT) to combine keywords in a database search allows you to narrow or expand your search. To build a complex search using two or more Boolean operators, you will need to learn the advanced technique of nesting (see Creating a Search Query for more details).

In the Venn diagrams below, the area retrieved is the yellow (or most lightly shaded) area.


Use AND to narrow a search. BOTH terms must be present in any references you retrieve.

Example: global warming AND forests 


Use OR to expand a search. Your search will retrieve records with EITHER of the terms.

Example: ethanol OR methanol 

OR is most often used to combine synonyms or like terms.



Use NOT to exclude a term. Records with the first term will be retrieved, but any records with the second term will be eliminated.

Example: california NOT baja 

Wildcards and truncation symbols (* # ? !)

For terms that have variant forms of spelling or different possible endings.

Examples: child* for child, children, childhood, childish, etc.