Hi Dorraj
DorraJ Oet wrote:
>
> I need clarification for something basic.
>
> We know that 2 independent samples Ttest has the following assumptions
>
> 1. The scale follows normal distribution.
> 2. The 2 categories in the categorical variable is mutually exclusive.
> 3. The homogenity of variance assumption has to be met.
>
>
> My question is as follows
>
> 1. Assuming that the normality is met, what happen if the
> homogenity of variance assumption failed? Do we go to a
> nonparametric test say Mann Whitney or do we just use the
> variance not assumed Ttest Statistics in the Ttest table?
>
Although not a lot of people knows that, MannWhitney's test is very
sensitive to differences in spread (heterogeneity of variances).
Therefore, it is as bad as a Ttest when homogeneity of variances is not
met. You must use the "variances not assumed" (Welch test is the name,
BTW) Ttest.
>
> 1. I had a professor once commented that he will use the variance
> not assumed Ttest Statistics and that Statistics is the
> calculation when the variance is not pooled but for separated
> categories. He even mentioned that going from Parametric to
> Nonparametrice approach is not correct. Is he right by
> commenting these?
>
Yes, he is indeed (see above). "Distributionfree" is a misnomer.
Nonparametric tests can have some conditions concerning the
distribution of the data. For instance, signed ranks Wilcoxon test needs
symmetrical distributions, MannWhitney's U and KruskalWallis tests are
sensitive to differences in shape and or spread of the groups being
compared...
HTH,
Prof. Marta Garc¨ŞaGranero

For miscellaneous SPSS related statistical stuff, visit:
http://gjyp.nl/marta/
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