A measure of consistency where a test is split in two and the scores for each half of the test is compared with one another. If the test is consistent it leads the experimenter to believe that it is most likely measuring the same thing. This is not to be confused with validity where the experimenter is interested if the test measures what it is suppose to measure. A test that is consistent most likely is measuring something; the experimenter just does not know what that "something" is. This is why it is said that reliability sets the ceiling of validity.