It's definitely good to understand the underlying math, and it's a good article on "this is how to translate a formula into Ruby code", but given that the article is positioned as "We needed to solve this problem, and this is how we solved it", it seems like it'd make more sense to focus on solving it with the least work and the best performance, which is why I mentioned GSL.

It's great to see other people doing statistical work in Ruby, though, so please don't let my criticism keep you from continuing to do it! :)