coefficient of reliability

n. A numeric index which reflects the stability of a test score or the relative proportion of true score and random error within a test score. The most important coefficient of reliability is a test-retest correlation coefficient, which estimates the temporary stability of a test. Kuder-Richardson 20, coefficient alpha, and split-half reliability coefficients are all measures of the internal consistency of a test, which is important when unidimensionality is an issue or when the dimension being measured is not expected to have temporal stability, as in a mood scale. The alternate forms reliability correlation coefficient is rarely used as it is difficult to ensure alternate forms are actually equivalent. 

▶ See also RELIABILITY COEFFICIENT