# File lib/minitest/benchmark.rb, line 198
    def fit_error xys
      y_bar  = sigma(xys) { |x, y| y } / xys.size.to_f
      ss_tot = sigma(xys) { |x, y| (y    - y_bar) ** 2 }
      ss_err = sigma(xys) { |x, y| (yield(x) - y) ** 2 }

      1 - (ss_err / ss_tot)
    end