def calc_measurement_error(self, tangents):
        '''
        formula for measurement error
        sqrt ( (sum(1, n, (k_i - <k>)**2) / (n*(n-1)))
        '''

        if len(tangents) < 2:
            return 0.0

        avg_tan = float(sum(tangents) / len(tangents))
        numerator = float()
        for i in tangents:
            numerator += (i - avg_tan) * (i - avg_tan)

        return math.sqrt(numerator / len(tangents) / (len(tangents) - 1))