Without normative data, the information obtained regarding a person is meaningless. Norms provide a means of assessing a person's relative standing in comparison to others. They include comparing IQ and grade equivalence. Within-group norms are used to evaluate a person's performance in terms of a similar comparison group. For example, comparing a child's performance to others his or her age, or comparing a schizophrenic patient to other schizophrenics.
Reliability refers to the consistency of scores obtained by a person when re-examined.
The degree to which a test is reliable defines the accuracy with which it assesses the person. Although generally useful, this type of reliability has limitations. Some behaviors fluctuate extensively and it is necessary to take this into consideration when using test-retest reliability. More seriously, are issues related to the impact practice may have on a test. This type of reliability is useful for measures of stable personality traits, but not for measures of aptitude, where practice severely impacts performance on future administrations.