calculating standard deviation in a crosstab

[johangel]

Hi everyone,
I have a crosstab:
the row = education discipline
the colums: the points going from 0 to 20, so there are 21 colums called 'points'
the body: number of students who get the different scores
f.i.
biologie: 2 students with 0 points, 1student  with 1point,6students  with 3points,....1 student with 20 points ( ;D)
total of students for education disipline and average for education disipline is no problem,but how to calculate standarddeviation.?
I made this sd:

"total(power(points - average);2) for education disipline) / total students" give me a result, but not the result I expect, it is to high. (the aggregate properties of the data item sd  is set 2X to none)

The built in function standard deviation is not working, it gives the same result for every colum,whatever I do
"standard_deviation(points) for education discipline"
"standard_deviation(points) for all education discipline"
"standard_deviation(points) for any education discipline"
"distinct standard_deviation(points) for education discipline"
Thank you