test of equivalence for two independent samples, base on t-test
Parameters:
low, upp : float
equivalence interval low < m1 - m2 < upp
usevar : string, ‘pooled’ or ‘unequal’
If pooled, then the standard deviation of the samples is assumed to be
the same. If unequal, then Welsh ttest with Satterthwait degrees
of freedom is used
Returns:
pvalue : float
pvalue of the non-equivalence test
t1, pv1 : tuple of floats
test statistic and pvalue for lower threshold test
t2, pv2 : tuple of floats
test statistic and pvalue for upper threshold test