Test–retest reliability is one way to assess the consistency of a measure. The reliability of a set of scores is the degree to which the scores result from systemic rather than chance or random factors. Reliability measures the proportion of the variance among scores that are a result of true differences. True differences refer to actual differences, not measured differences. That is, if you are measuring a construct such as depression, some differences in scores will be caused by true differences and some will be caused by error. For example, if 90% of the differences are a result of systematic factors, then the reliability is .90, which indicates that 10% of the variance is based on chance or random factors. Some examples of chance or ...