def anova_table(aov):
    """ Add effect siz measure to ANOVA table.

    Statsmodel's ANOVA table does not provide any effect size measures to tell
    if the statistical significance is meaningful. Here eta-squared and omega-squared are calculated.
    Omega-squared is considered a better measure of effect size since it is unbiased
    in it's calculation by accounting for the degrees of freedom in the model.
    """
    aov['mean_sq'] = aov[:]['sum_sq']/aov[:]['df']
    aov['eta_sq'] = aov[:-1]['sum_sq']/sum(aov['sum_sq'])
    aov['omega_sq'] = (aov[:-1]['sum_sq']-(aov[:-1]['df']*aov['mean_sq'][-1]))/(sum(aov['sum_sq'])+aov['mean_sq'][-1])
    cols = ['sum_sq', 'df', 'mean_sq', 'F', 'PR(>F)', 'eta_sq', 'omega_sq']
    aov = aov[cols]

    return aov