Hi all,
i have a Before-After-Control-Treatment (2x2) design, resulting in two independent groups of scale data (samples in two water streams, in one a treatment has been installed at some point, both streams (two physical different locations) were sampled before and after at various occasions). i would like to test for differences in distribution, the effect of this treatment.
My data are not-normal distributed, and some of the group combinations violate the assumption of equality of variance, as the output of the Modified Levene's test testified.
Initially I have performed Mann-Whitney U tests on the groups so far, with several outputs resulting a significant difference across the treatment.
But MW-U assumes equality of variance.
1) How sensitive is MW-U to the violation of equality of variances?
2) To determine an effect size of the treatment, for instance eta squared, using the Z statistic of MW-U, is equality of variance needed?
3) Are there any alternative post-hoc tests similar to the univariate Games-Howell test (which does not assume equality of variance), that can be run on non-parametric 2x2 designs?
ps: i am working in SPSS
Many Thanks,
Taco
i have a Before-After-Control-Treatment (2x2) design, resulting in two independent groups of scale data (samples in two water streams, in one a treatment has been installed at some point, both streams (two physical different locations) were sampled before and after at various occasions). i would like to test for differences in distribution, the effect of this treatment.
My data are not-normal distributed, and some of the group combinations violate the assumption of equality of variance, as the output of the Modified Levene's test testified.
Initially I have performed Mann-Whitney U tests on the groups so far, with several outputs resulting a significant difference across the treatment.
But MW-U assumes equality of variance.
1) How sensitive is MW-U to the violation of equality of variances?
2) To determine an effect size of the treatment, for instance eta squared, using the Z statistic of MW-U, is equality of variance needed?
3) Are there any alternative post-hoc tests similar to the univariate Games-Howell test (which does not assume equality of variance), that can be run on non-parametric 2x2 designs?
ps: i am working in SPSS
Many Thanks,
Taco
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