Psychological researchers can never study all of the cases in which they are interested, such as all adults or all obsessive-compulsives, because these populations are much too large. They must use...

Psychological researchers can never study all of the cases in which they are interested, such as all adults or all obsessive-compulsives, because these populations are much too large. They must use relatively small samples drawn from the specifi ed populations. One or two studies never disprove (or prove) a theory, because studies using different samples might have yielded different results. A substantial number of studies that disconfi rm a theory would be another story, but it is not easy to obtain such consistency in psychological research.