I wrote a blog article about this (contains a bit more math) http://www.datagenetics.com/blog/december32012/index.html http://www.datagenetics.com/blog/december

I wrote a blog article about this (contains a bit more math) http://www.datagenetics.com/blog/december32012/index.html http://www.datagenetics.com/blog/december32012/index.html A slightly different variant, is how to make it totally random selection http://www.datagenetics.com/blog/june52013/index.html http://www.datagenetics.com/blog/june52013/index.html