Dr. Mike, a local medical doctor in the greater Philadelphia area, frequently appears on the 104.5, Preston and Steve radio show. Dr. Mike is highly esteemed in his field and provides up-to-date medical information for 104.5 listeners. He often calls in to the radio show to provide answers to either the radio staffs' or radio listeners' questions. Sometimes he recommends specific remedies and products, and Valerie, a regular listener of the Preston and Steve show, trusts what he says or recommends because he doesn't seem to have an apparent motive to mislead listeners. Which characteristic does Dr. Mike possess