It is important to note that measurement validation refers not to the properties of a particular instrument but rather to the conclusions made as a result of its use. In other words, although we speak of a valid instrument, what we really mean is an instrument from which we can make valid inferences. In the latter, we relate the items in the instrument to the constructs which we are attempting to measure. Reliability Reliability may be defined as the extent to which a measurement process or instrument consistently measures factors regardless of external conditions such as time or setting. Similarly, a measurement instrument may be said to be reliable if it yields the same results in different conditions of administration, at different times, and in different places.
Reliable measurement is relatively free from the extraneous influences of administration context.
The goal of a reliability study is to amass evidence demonstrating the reliability of a measurement instrument. Our approach to a reliability study would typically involve test-retest reliability and internal consistency statistics.