Relevance
Does this item address the issue that you are researching?  A source is relevant if the majority of it is tied to the subject you are researching.  Today many students search on databases or the Internet, and often the items found by such searches are not really about the desired topic.  A search using the term “AIDS,” for example, might find an article such as “Training Teachers How to Use Teaching Aids.”  While the word “AIDS” was part of the title, this is obviously not an article about the AIDS disease.