Researchers and clinicians using Jacobson and Truax's index to assess the reliability of change in patients, or its counterpart by Chelune et al., which takes practice effects into account, are confused by the different ways of calculating the standard error encountered in the literature (see the discussion started in this journal by Hinton-Bayre). This article compares the characteristics of (1) the standard error used by Jacobson and Truax, (2) the standard error of difference scores used by Temkin et al. and (3) an adaptation of Jacobson and Truax's approach that accounts for difference between initial and final variance. It is theoretically demonstrated that the last variant is preferable, which is corroborated by real data.