statsmodels.stats.oneway.equivalence_scale_oneway(data, equiv_margin, method='bf', center='median', transform='abs', trim_frac_mean=0.0, trim_frac_anova=0.0)[source]

Oneway Anova test for equivalence of scale, variance or dispersion

This hypothesis test performs a oneway equivalence anova test on transformed data.

Note, the interpretation of the equivalence margin equiv_margin will depend on the transformation of the data. Transformations like absolute deviation are not scaled to correspond to the variance under normal distribution.

datatuple of array_like or DataFrame or Series

Data for k independent samples, with k >= 2. The data can be provided as a tuple or list of arrays or in long format with outcome observations in data and group membership in groups.


Equivalence margin in terms of effect size. Effect size can be chosen with margin_type. default is squared Cohen’s f.

method{“unequal”, “equal” or “bf”}

How to treat heteroscedasticity across samples. This is used as use_var option in anova_oneway and refers to the variance of the transformed data, i.e. assumption is on 4th moment if squares are used as transform. Three approaches are available:

“unequal”Variances are not assumed to be equal across samples.

Heteroscedasticity is taken into account with Welch Anova and Satterthwaite-Welch degrees of freedom. This is the default.

“equal”Variances are assumed to be equal across samples.

This is the standard Anova.

“bf”Variances are not assumed to be equal across samples.

The method is Browne-Forsythe (1971) for testing equality of means with the corrected degrees of freedom by Merothra. The original BF degrees of freedom are available as additional attributes in the results instance, df_denom2 and p_value2.

center“median”, “mean”, “trimmed” or float

Statistic used for centering observations. If a float, then this value is used to center. Default is median.

transform“abs”, “square” or callable

Transformation for the centered observations. If a callable, then this function is called on the centered data. Default is absolute value.

trim_frac_meanfloat in [0, 0.5)

Trim fraction for the trimmed mean when center is “trimmed”

trim_frac_anovafloat in [0, 0.5)

Optional trimming for Anova with trimmed mean and Winsorized variances. With the default trim_frac equal to zero, the oneway Anova statistics are computed without trimming. If trim_frac is larger than zero, then the largest and smallest observations in each sample are trimmed. see trim_frac option in anova_oneway

resultsinstance of HolderTuple class

The two main attributes are test statistic statistic and p-value pvalue.