Calculate the Z-Score between this alternative and the project control.

        Statistical formulas based on:
        http://20bits.com/article/statistical-analysis-and-ab-testing

def z_score(self):
        """
        Calculate the Z-Score between this alternative and the project control.

        Statistical formulas based on:
        http://20bits.com/article/statistical-analysis-and-ab-testing
        """
        control = VariantStat(self.experiment.control, self.experiment)

        alternative = self

        if control.name == alternative.name:
            return 'N/A'

        conv_c = control.conversion_rate
        conv_a = alternative.conversion_rate

        num_c = control.participant_count
        num_a = alternative.participant_count

        if conv_c == 0 or conv_a == 0:
            return 0

        numerator = conv_a - conv_c

        frac_c = (conv_c * (1 - conv_c)) / float(num_c)
        frac_a = (conv_a * (1 - conv_a)) / float(num_a)

        if frac_c + frac_a == 0:
            # square root of 0 is 0, so no need to calculate
            return 0
        elif frac_c + frac_a < 0:
            # can't take a square root of a negative number,
            # so return 'Invalid'
            return 'Invalid'

        return numerator / math.sqrt((frac_c + frac_a))