Test-retest reliability is the degree to which test scores remain unchanged when measuring a stable individual characteristic on different occasions.
Test-retest reliability measures the stability of the scores of a stable construct obtained from the same person on two or more separate occasions. Reliability concerns the degree to which scores can be distinguished from each other, despite measurement error. In the case of test-retest assessment, intraindividual response variability is used to estimate measurement error (Hays et al. 1993).
According to classical test theory (CTT), reliability (R) is defined as the ratio of true score variance (σt 2 ) to total score variance, which is the sum of the true score variance and the error variance (σε 2 ).
