Reliable change criterion calculator

Mounted by Chris Evans in 1998, last updated 20.v.04. Ooops, and redirection added 14.ix.24 & tweaked 15.ix.24!

REDIRECTING you ... and some history

Wow, over twenty years on I am, belatedly, redirecting you to https://shiny.psyctc.org/apps/RCI1/. That actually replaced the defunct perl cgi-lib app some way back and I fixed the parent page to redirect things but a kind person has just alerted me that I hadn't redirected this page as I assumed people wouldn't get to it: wrong Christopher, the internet has a bigger memory than any elephant!

You might also want to look at https://shiny.psyctc.org/apps/RCI2/ That gives a "plausible interval" around the point RCI value. That is roughly equivalent to a confidence interval and is based on the standard error of the standard deviation. It can be a very useful cautionary note about how imprecisely the RCI is likely to have been estimated: not very precisely unless your n is very large.

The rest of this is purely here for historical completeness ...

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:

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.