Hi all,
i have a BeforeAfterControlTreatment (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 notnormal 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 MannWhitney U tests on the groups so far, with several outputs resulting a significant difference across the treatment.
But MWU assumes equality of variance.
1) How sensitive is MWU 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 MWU, is equality of variance needed?
3) Are there any alternative posthoc tests similar to the univariate GamesHowell test (which does not assume equality of variance), that can be run on nonparametric 2x2 designs?
ps: i am working in SPSS
Many Thanks,
Taco
i have a BeforeAfterControlTreatment (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 notnormal 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 MannWhitney U tests on the groups so far, with several outputs resulting a significant difference across the treatment.
But MWU assumes equality of variance.
1) How sensitive is MWU 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 MWU, is equality of variance needed?
3) Are there any alternative posthoc tests similar to the univariate GamesHowell test (which does not assume equality of variance), that can be run on nonparametric 2x2 designs?
ps: i am working in SPSS
Many Thanks,
Taco
Attachments

22.4 KB Views: 2