Sapmle size calculation

Hi I'm trying to conduct a sample size calculation for my final year project.
The Unified Parkinson's Disease Rating Scale is my main outcome measure.
I found a MCID for this scale to be 8 points.
Could anyone give me advice on how to get the standard deviation? Are there set highest and lowest typical values for this outcome measure?
Or how is the standard deviation collected from previous studies?
If anyone has any advice for me this would be great!

hi I may be a bit late with this but you can down load an excellent (and simple) sample size calculator from
PowerSampleSize < Main < Vanderbilt Biostatistics Wiki
the SD of your primary outcome can be obtained from a previous publication that used the outcome measure.

- ProEditor