Feature Request: Wilson Score method

[Don Leatham]

I've noticed that sorting on votable records with 'plusminus' often leaves me with strange results, depending on the number of votes one record may have in relation to another. Posts with a lower percentage of positive votes could be rated higher due to fact they have been around longer and have a larger pool of votes. I saw an article that clearly describes this problem and offers up a solution to it.

http://evanmiller.org/how-not-to-sort-by-average-rating.html

He bases the answer on a mathematical calculation called a "Wilson Score". The Ruby code he provides with explanation is as follows:

require 'statistics2'

def ci_lower_bound(pos, n, confidence)
    if n == 0
        return 0
    end
    z = Statistics2.pnormaldist(1-(1-confidence)/2)
    phat = 1.0*pos/n
    (phat + z*z/(2*n) - z * Math.sqrt((phat*(1-phat)+z*z/(4*n))/n))/(1+z*z/n)
end

pos is the number of positive ratings, n is the total number of ratings, and confidence refers to the statistical confidence level: pick 0.95 to have a 95% chance that your lower bound is correct, 0.975 to have a 97.5% chance, etc. The z-score in this function never changes, so if you don't have a statistics package handy or if performance is an issue you can always hard-code a value here for z. (Use 1.96 for a confidence level of 0.95.)*

This would be a nice add to plusminus as a way to accurately measure popularity. I would volunteer, but I'm a part-time amateur at best, and no one (including me) would want my code going into production. (I'm a business person that does very basic MVPs.)

[Brady Bouchard]

Already in the codebase, just hasn't been pushed yet ;)

[Brady Bouchard]

Pushed and released as 0.5.4... please feel free to edit the README and send a pull request if you have a chance!