A good research project usually involves using multiple sources and different kinds of information.
A common mistake students make when writing papers is to rely on one or two sources and to only use one type of source.
Number of Sources:
If you rely on one or two sources, you probably are not going to get very complete coverage.
Different Types of Sources:
If you only use one type of source-only magazines or only web sites, for example--you'll probably miss lots of useful information