The formula for the standard eror of change is:

SD1*sqrt(2)*sqrt(1-rel)where SD1 is the initial standard deviation

sqrt indicates the sqare root

rel indicates the reliability

The square root of two terms comes in because you are looking at the difference between two unreliable measurements and was left out of the original paper:

Jacobson, N. S., W. C. Follette & D. Revenstorf (1984). Psychotherapy outcome research: methods for reporting variability and evaluating clinical significance. Behavior Therapy 15: 336-352

and corrected in:

Christensen, L. & J. L. Mendoza (1986). A method of assessing change in a single subject: an alteration of the RC index. Behavior Therapy 17: 305-308.

The criterion that is always used is the 95% criterion, i.e. a value such that change greater than this, improvement and deterioration, would only occur by unreliability of measurement alone in less than 5% of times that two measurements are made on the same person.

The following form calculates this for you: