Use NOT to exclude a term. Records with the first term will be retrieved, but any records with the second term will be eliminated.
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This method uses subjects as 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 keywords in a literature database or catalog as shown in the following example:
Start by constructing a search strategy using the following four steps.
The following is an example of a research interest/topic.
I am interested in finding research on the effect of sedimentation on the survival of salmon eggs.
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.
In the following topic example, the four underlined words – sedimentation, survival, salmon, eggs – are unique concepts.
effect of sedimentation on the survival of salmon eggs
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:
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. In a database record they may be called subject terms, descriptors, etc. The following example is of the Subject Terms search in EBSCOhost's Academic Search Premier database.
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 <-- Sporolithon ptychoides
For taxonomic concepts use both common and scientific names of organisms. Using both will normally increase the number of citations retrieved.
salmon or Oncorhynchus
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 may be able to use wildcard symbols (e.g., *, #, ?, +) 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
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*)
Sometimes it is useful to conduct a preliminary search before beginning your main search. This search serves two purposes:
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 subject terms or descriptors, which commonly appear as part of each citation. 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 OR-ing 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:
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 will grow and/or be refined.
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.
Use OR to expand a search. Your search will retrieve records with EITHER of the terms.
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.
For terms that have variant forms of spelling or different possible endings.
Examples: child* for child, children, childhood, childish, etc.