All tests are designed to measure something; hopefully something specific. If the test does indeed measure what it is intended to measure, then we can say that the test is valid (or has validity). In psychology, tests are usually judged according to their validity and their reliability (if the test produces similar results each time the test is taken). Tests that are valid are also reliable. However a test might be reliable without it being valid. For example, let's say you go to class and your teacher says that he or she has learned that the weight of your head determines the likelihood of you getting cancer. So, each day at the beginning of class the teacher weighs everybody's head on a scale. This continues for one week. Across each day of the week, the weight of your head is going to remain approximately the same; thus we can conclude that the test is reliable. However, is this test valid; does it measure what it is intended to measure (that the weight of your head predicts the likelihood of you getting cancer)? The answer is....NO! So, although the test is reliable, it is not necessarily valid (it does NOT measure what it was designed to measure).
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