Use specialized encyclopedias, handbooks and scholarly books for excellent overviews and syntheses of possible topics. Use bibliographies at the end of encyclopedia articles for further inquiry.
How to find these sources?
Start by phrasing your subject or general topic in the form of a question.
Then ask yourself further questions about your topic:
Next, look for resources which provide background information. Some selected general and specialized subject sources can help narrow the topic.
Remember, there are two layers of research:
1) a broad search to discover resources and to read some background information
2) specific searches for information once you've focused your topic.
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You may also want to consult ideas on the University of Richmond's Writer's Web.
Link to Narrowing Your Topic: 5 Quick Tips