Chapter 2: why research is best and how to find it. There are three sources of evidence of people"s beliefs: experience, intuition, authority. Empirical research is a superior source of evidence. He may have noticed that some of his patients recovered and some died. That is every time a patient recovered from a yellow fever after bloodletting, it supported his theory that the treatment worked. If patient died, then they were classified as too sick to recover . One problem was that the doctor never set up a systematic comparison. Creating a table or matrix would have been beneficial in order to compare off of his outcomes. A confound occurs when you think one thing caused an outcome but in fact other things changed, too, so you are confused about what the cause really was. For a personal experience its hard to isolate variables.