Explain the difference between reliability and validity

What will be an ideal response?

a . Reliability is focused on consistency in measurement. Consistency can be measured in several ways. With test-retest reliability, the consistency is temporal in nature. A psychologist measures a characteristic at one time, and then measures it at a later time to see if the measure is stable across times. Alternately, consistency can be measured as "internal consistency" of a measure. If a measure has multiple items on it, all of them purporting to measure the same thing, the items should be highly inter-correlated. Validity, however, focuses more on the degree to which a survey scale measures what it claims to measure. For example, face validity is the degree to which a scale appears on the face of it to measure what it says it measures. Of course, this is not alone sufficient to establish validity. Convergent validity speaks to a measure correlating with other, related measures, while divergent validity speaks to a measure failing to correlate with unrelated measures. Measurement validity deals with whether or not a measure is predictive of outcomes it should be able to predict. An instrument cannot be valid unless it first is reliable. If a measure is inconsistent (either temporally or internally) then we cannot know for certain what exactly is being measured.