dacohen/Rsquared

module Enumerable
       def sum
              return self.inject(0){|acc, i| acc+i}
       end

       def mean
              return self.sum/self.length.to_f
       end

       def variance
              varsum = self.inject(0){|acc, i| acc + (i - self.mean)**2}
       return(varsum/(self.length.to_f-1.0))
       end

       def popvariance
              return self.variance*((self.length.to_f-1.0)/self.length.to_f)
       end

       def popstddev
              return Math.sqrt(self.popvariance)
       end

       def stddev
              return Math.sqrt(self.variance)
       end

       def skew
              thirdsum = self.inject(0){|acc, i| acc + (i - self.mean)**3}
       thirdmoment = thirdsum/self.length.to_f
       return thirdmoment / (self.popvariance)**(3.0/2.0)
       end

       def kurtosis
              fourthsum = self.inject(0){|acc, i| acc + (i - self.mean)**4}
       fourthmoment = fourthsum/self.length.to_f
       return (fourthmoment / (self.popvariance)**2)
       end

       def std
              result = []
              (0..self.length-1).each do |i|
           result[i] = (self[i] - self.mean)/self.stddev
       end
       return result
       end
end