These databases index journals articles....
Boolean Searching allows you to combine words or phrases using the words AND, OR, NOT. These are called Boolean Operators. One way to think about how they will retrieve search results for you, is in the diagram below:
Using the Advanced Search feature inside of Google Scholar, can also assist you in performing Boolean Searching. It can help you to make determinations when to search whole phrases of information. For example: Crohn's disease and "Crohn's disease" are two different ways to search. By not including quotations you are essentially telling the search engine to find any result with these two words. The words will not necessarily be together, and will return substantially more results than if you were to combine the words with quotations. Using quotations keeps your search terms together and searches by an exact phrase.
