Whenever anything is measured whether in the physical biological or behavioral sciences there is some possibility of chance error.

The other one is the consistency of results upon re-testing. consistency of results throughout the test when administered once. Reliability has two important factors: internal consistency i.e. These errors do not just come from the instability of the subject’s performance or because of chance factors but since individuals do not perform with complete consistency upon all times and chance factors cannot be completely eliminated the reliability for psychological tests are the result of interaction through individual differences, defects of the instrument and chance determinants. It is the ability to yield consistent results from one set of measures to another it is the extent to which the obtained test scores are free from such internal defects as will produce errors of measurement inherent in the items and their standardization. To begin with, reliability refers to the consistency of scores obtained by the same persons when they are re-examined with the same test on different occasions, or with different sets of equivalent items or under other variable examining conditions. Apart from its above stated purpose, they are also the requisite for Clinical Assessment and Diagnosis. Reliability and Validity are considered linchpin to any psychological measurement which lead to interpretability of individual and group scores. To begin with there are many desirable properties of Psychological measures viz: Standardization, population norms to name a few.

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Reliability and validity of measurement