Reliable change criterion calculator

Mounted by Chris Evans in 1998, last updated 20.v.04
The formula for the standard eror of change is:
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:

Initial standard deviation (use full stop "." for decimal point)

Reliability (use full stop "." for decimal point)

Press to submit your query.

The program would not have been possible without Steven E. Brenner's excellent Perl CGI-LIB library nor the whole WWW endeavour and the wonders of Perl.