reliability

n. The degree to which a measure of a psychological characteristic gives similar results under different conditions. If a bathroom scale indicates that a person weighs 150 lb. on Tuesday and 250 lb. on Thursday, we are likely to conclude that it is broken; its results are unreliable. Similarly, if a personality scale suggests that one is an extrovert on one day and an introvert on the next, we may suspect that the test is not fully reliable. There are several different kinds of reliability that must be distinguished, including internal consistency, retest, and interrater reliability. Establishing 
reliability is one of the first tasks in creating a psychological measure, because acceptable reliability is generally necessary for psychological measurement. However, reliability is not sufficient, because a reliable measure can be consistently wrong, like a scale that repeatedly reports a weight of 150 lb. for a person who actually weighs 250 lb. – RRM