How to Make aggragate Function? Standard Deviation?

Hi all,

I am just starting to write functions in plpgsql.  I have made some
simple functions but now I need a standard deviation function.  The
documentation was not too clear so does anyone have any suggestion on
how to begin?  I can do this in plpgsql or do I need to do this in C?

thanks,

Phillip J. Allen
Consulting Geochemist/Geologist
Lima Peru
e-mail: paallen@attglobal.ent