'sumX = x1 + x2 + ... + xn
avgX = sumX / n
sumSquareX = x1^2 + ... + xn^2
SD = sqrt([sumSquareX - (n * (avgX ^ 2))] / (n - 1))'

def _get_list_sd(self, data):

    sum = sqsum = 0
    n = len(data)
    for i in data:
        sum += float(i)
        sqsum += (float(i) ** 2)
    avg = (sum / n)
    if ((avg == 0) or (n == 1) or ((sqsum - (n * (avg ** 2))) <= 0)):
        return '0.0'
    return ('%.3f' % (((sqsum - (n * (avg ** 2))) / (n - 1)) ** 0.5))
