All sources must be published materials. Use only scholarly/peer reviewed works such as journal articles, books, reports, etc.
There are five criteria to consider when evaluating the credibility of information:
Authority: Look at the authors of the text. Who is the author(s)? Are they experts in the field/the topic? Have the authors been cited by other authors in the field?
Currency: This examines when the article/book were published. Is the publication date right for your needs? Do you need current or historical information? The age of the information cited in the article/book?
Content: How relevant is the information to your topic? Is the target audience appropriate? Is the article/book written in academic or scholarly language?
Accuracy: How adequately the authors explained the research methodology used? Can you verify the accuracy in other sources? Is it peer-reviewed?
Bias: This examines if the research was sponsored by an organization? (Note: Organizations may have motives). Is the author trying to sell or push a product or service? Do other authors agree with the author's point of view?
For more information on how to evaluate the source of information, see the library research guide -- Evaluating Sources.