Add support for Krippendorff's alpha reliability coefficient

[agarie]

(Original: clbustos/statsample#13)

(First of all, thanks so much for this package!
While it took me some time to get used to it, it's super handy even for simple statistics in business applications.)

I might lack the knowledge to combine the pieces of this framework, but I'm missing explicit support for measuring reliability between raters, e.g. Krippendorff's alpha.

R provides a nice interface for this here.

Thanks again for your efforts and time in supporting this project!

[agarie]

@ludwigschubert also replied:

Yes.

The Original paper by Prof. Krippendorf is available here.
Together with a colleague he also provided a (to me unreadable) implementation in SAS here.

Instead, I'd recommend working of Matlab code and the wikipedia article on the subject.
I'm currently using R's package "irr"'s implementation, which can be found here - I extracted the specific file for you in this GIST.

Let me know if there's anything else I can help with - unfortunately I had about one introductory class in statistics, so the finer points of Krippendorf's alpha are entirely lost on me.