# File lib/camping-abingo.rb, line 595
        def zscore
                if alternatives.size != 2
                  raise "Sorry, can't currently automatically calculate statistics for A/B tests with > 2 alternatives."
                end

                if (alternatives[0].participants == 0) || (alternatives[1].participants == 0)
                  raise "Can't calculate the z score if either of the alternatives lacks participants."
                end

                cr1 = alternatives[0].conversion_rate
                cr2 = alternatives[1].conversion_rate

                n1 = alternatives[0].participants
                n2 = alternatives[1].participants

                numerator = cr1 - cr2
                frac1 = cr1 * (1 - cr1) / n1
                frac2 = cr2 * (1 - cr2) / n2

                numerator / ((frac1 + frac2) ** 0.5)
        end