Masters and Johnson used direct observation for their research on sexual response and physiology. This method is the least frequently used because it is difficult to find participants who are willing to come into the laboratory to have sex while researchers monitor their bodily functions. However, if direct observation can be done, it does provide information that cannot be obtained elsewhere. Researchers can actually monitor behavior as it happens, which gives the results more credibility. A man may exaggerate
the number of erections per sexual episode in a self-report, but he cannot exaggerate in a laboratory.
Direct observation is much more expensive than any of the other methods and may not be as generalizable, because it would be impossible to gather a random sample. In addition, direct observation focuses on behaviors and, as a result, ignores feelings, attitudes, or personal history.