Relevance is the most important criterion for evaluation. If you choose to use no other criteria, you should select source material based on its pertinance to your topic. If your search strategy was good, a large percentage of your work is already done. (See Lesson 4.)
What you will need to do now, is sort through your search results and weed out false drops:
For example, in the Library Catalog a keyword search for george washington and biography retrieves 53 items. Most of them are about the first president of the United States, however, eight of them are actually about George Washington Carver, seven other items are about various people whose first and middle names were George and Washington, and a few were retrieved because they mentioned two different people, one with the name of George and the other with the name of Washington. The ambiguity in keyword searching for proper nouns can easily be fixed by using subject headings instead of keywords. Lesson 4 discusses subject headings in more detail.
Develop a process for examining your search results.
Some records will clearly be irrelevant under any circumestances. Other items may be relevant in one situation and not in another.
For example, among the biographies on George Washington retrieved in the above search were several children's books. These would not generally be considered authoritative sources (discussed next), however if one were writing an analysis of how the first president of the United States is portrayed in children's literature, these books would be viable sources.
In essence, relevance is determined by the source document's relationship to your topic.