Search Techniques: Combining search terms

                           Fig. 1                                                                                                                           Fig. 2

The three most commonly used operators are AND, OR, NOT.  These are known as Boolean operators. They can be used to broaden or narrow a search and to exclude unwanted search terms and concepts.

It is possible to use a number of different keywords or search terms in a single search, by using operators. 

Operators link your search terms and define the relationship between them. This enables more accurate searching and therefore more relevant results being returned. It also saves you time as you don't have to carry out numerous similar searches where just one or two search terms are changed each time.

Use AND to narrow your search.

Using AND between your search terms narrows your search as it  instructs the database that all your search terms must appear (in any order).

For example: semiotics AND drama
                      - will only return results where both words are present

Because all search terms must be present, using AND makes the search more focused. 

In some (but not all) databases and search engines the AND is implied so if you enter multiple words the database will search for results which contain all/both words.

Use OR to broaden your search.

Using OR between search terms broadens your results as any or all of your search terms can be present.

For example: religion OR belief 
                      - will return results where either one or both of these words are present

It is extremely useful for finding synonyms or related concepts.

Using OR  enables you to carry out a number of similar searches in one go, saving you time.

Use NOT to narrow your search.

Using NOT narrows your search as it instructs the database to ignore results that contain particular words.

For example: social media NOT twitter
                      - will return results that include social media but do not  include twitter

NOT  tends to be used less than the And and OR operators. 

Use it with care as you may exclude useful articles which cover a range of topics of relevance to you.

Databases usually process the Boolean operators in a logical order.

They typically recognise AND as the primary operator and will connect concepts with AND together first.

If you use a combination of AND and OR operators in one search, use parentheses (brackets) to group the OR words together so that your search is processed correctly.

For example:

If you want to find information on cloning humans and cloning sheep, use the following instruction:

cloning AND (sheep OR human)  - This will search for cloning AND sheep as well as cloning AND human

If you do not use the parentheses and search using the following:

cloning AND sheep OR human, your search will be processed as:

• cloning AND sheep as one search
• OR human as a secondary search - This means that your search results containing human would not be linked in any way to cloning.

Many databases offer the functionality to separate out your search terms, and this saves you having to add the parentheses. Sometimes you will have to access the Advanced Search for these options.

In this example from the Web of Science database, you can use the Basic search form to add multiple search lines by clicking on Add Another Field.