def _confidence(series):
    """
    95% confidence intervals defined as:
        1.96 * standard deviation from the mean / sqrt(number of items)

    1.96 assumes a normal distribution.

    https://www.itl.nist.gov/div898/handbook/prc/section1/prc14.htm
    http://sphweb.bumc.bu.edu/otlt/MPH-Modules/BS/BS704_Confidence_Intervals/BS704_Confidence_Intervals_print.html
    """
    return 1.96 * series.std() * (series.size ** -0.5)