4. Using OR & AND
LITERATURE SEARCHING
Conducting a topic-based search of the Ovid SP
databases available via the NHS Scotland Knowledge
Network
These guides are designed to facilitate self-study, or to accompany a librarianled session on searching for information via the OvidSP databases.
Written by Seona Hamilton. Please contact your local or subject librarian for training in literature searching and
using online databases.
Last updated June 2011
1
Using AND and OR to join subjects (Boolean Logic)
About Boolean logic
Boolean logic was named after the mathematician George Boole. It is the method you use to
express your search question in a way that a computer database will understand.
While it would be great if you could just type in your search question and get back a list of relevant
references (this is called “natural language” searching), for most databases you have to split your
question into topics and then join them together using specific terms.
Boolean search operators
The three key operators in Boolean Logic which you use to join together search terms are AND,
OR and NOT. Most of the time you will just use AND and OR.
The main things to remember are that you use OR to get more, to increase your pool of references
and expand the scope of your search. You use AND to narrow down your search, to be more
specific and you usually get less references.
You can use AND and OR to combine subject headings or free text searches, or to combine
subject headings with free text searches.
Using AND
Pubic Symphysis AND Pregnancy
Includes all articles that are about Public Symphysis AND all articles that are about Pregnancy.
Excludes any article that is not about BOTH subjects.
Using AND usually gives you a smaller set of articles than either subject on its own.
Articles
about Pubic
Symphysis
Articles
about
Pregnancy
Public
Symphysis
AND
Pregnancy
e.g. Running this search in Medline would produce:
SEARCH TERM
Public Symphysis
Pregnancy
Pubic Symphysis AND Pregnancy
RESULTS
935
506445
261
Written by Seona Hamilton. Please contact your local or subject librarian for training in literature searching and using online
databases.
Last updated June 2011
2
Using OR
Tears OR Lacerations
Includes all articles that are EITHER about one subject OR another. Excludes any article that isn’t
about at least one of the topics.
Using OR usually gives you a larger set of articles than either subject on its own.
Articles
about Tears
Articles
about
Lacerations
Tears OR
Lacerations
e.g. Running this search in Medline would produce:
SEARCH TERM
Tears
Lacerations
Tears OR Lacerations
RESULTS
11530
2850
14268
Using NOT
It’s far less common to need to use NOT but it can be useful to exclude topics. For example, it can
be hard to single out studies that are purely to do with a paediatric population since the subject
heading Child/ is often included in studies where there may have only been one or two subjects
that were children and the rest adults.
To get round this you could include create your search strategy as usual and at the end add the
line [last set number] NOT exp Adult/ to exclude any references that included the subject heading
Adult.
Bear in mind however, that this might also exclude some studies that were of relevance to you, for
example those that were mainly children but included a subject that was old enough to come under
the subject heading Adult/ (e.g. an older teenager)
Example of search strategy using NOT:
SEARCH TERM
Leukemia
Child
Leukemia AND Child
Adult
(Leukemia AND Child) NOT Adult
RESULTS
143467
1062742
24759
3660064
15834
Written by Seona Hamilton. Please contact your local or subject librarian for training in literature searching and using online
databases.
Last updated June 2011
3
Combining OR and AND
When you combine search terms using more than one operator you need to use parentheses
(brackets) to tell the database in which order you want the operators to be processed.
(Tears OR Lacerations) AND Pregnancy
Articles about
tears or
lacerations in
pregnancy
Tears
Lacerations
Pregnancy
In the example above, you need to tell the database the you want it to first find the set that includes
all articles on tears OR lacerations, and then narrow down that set by only including the articles
that also are about pregnancy.
If you are not sure about using parentheses you can just use one operator at a time and then join
up your sets afterwards. The OVID databases allow you to use the set number to save you typing
your terms in again.
e.g. Instead of using the brackets, you could run the search above in the OVID databases as:
1.
2.
3.
Tears OR Lacerations
Pregnancy
1 AND 2
Most of the time you’ll be using OR at first to create big sets for each topic of your search, and then
using AND at the end to join up your sets and narrow them down into one set of (hopefully)
relevant references.
e.g. For a search about pain relief during childbirth you might build up this search first using OR:
A set of references about childbirth:
1. Labour OR childbirth OR delivery (freetext search of all the words that might be in the title or abstract of
articles to do with childbirth)
2. exp Labour, Obstetric/ (subject heading search for childbirth)
3. 1 or 2 (joining together the first two sets to get one big set of references about childbirth)
A set of references about pain relief:
4. Pain relief or epidural or analgesia (freetext search of all the words that might be in the title or abstract
5.
6.
of articles to do with pain relief)
exp Analgesia/ (subject heading search for pain relief)
4 or 5 (joining together the first two sets to get one big set of references about pain relief)
You might then finish your search by joining together your two big sets using AND
7. 3 and 6 (a set of references that are about both childbirth AND pain relief)
Written by Seona Hamilton. Please contact your local or subject librarian for training in literature searching and using online
databases.
Last updated June 2011
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