Javier is writing a research paper about the Vietnam War. He has decided to get some books from the library and to interview some people to use as sources. After doing a search for Vietnam, he comes across a book called America in Vietnam by Dr. Leslie Anderson at UCLA. He checks out the book. Later, he interviews his uncle, who served in Vietnam as an American soldier in 1970. What is one way for Javier to decide which source is more credible?