John Wilkes Booth was the son of prominent, wealthy actors. He, too, became an actor and was so popular, he was one of the first to have his clothes ripped off by fans. Hulton Archive/Getty hide caption
Psychology is the systematic investigation of the human mind, including behavior and cognition. It thus encompasses both normal and abnormal behavior. On the other hand, psychiatry is a branch of medicine dealing with mental illness.
Psychological tests, like intelligence tests, are evaluated on two broad dimensions: reliability & validity. Reliability is how consistent a test is, while validity is how accurate a test is. Tests can't be valid (that is, they can't be said to measure what they intend to measure) until they are first reliable.