Reliability & Validity

-- A scale cannot be both reliable and valid, since it would have to show the same weight each time a person weights themselves,  yet also it has to measure the actual weight of the person. It doesn't mean however, that it is actually valid or measures what it meant to.

-- What Moghaddam is trying to say is that even if an experiment could have a good reliability and a good internal validity, the external validity is important because it allows us to apply the results of a study to other groups, measures, persons, settings, and times.

--In a case of a survey or a study reliability is needed, but not sufficient to establish validity. We can get high reliability and low validity. This would happen when the wrong questions are asked over and over again, consistently yielding bad information. Also, if the results show large variation, they may be valid, but not reliable.