On Tue, 13 Apr 2010, Yvonnick Noel wrote:
Iurie,
Noel, thanks a lot. This will help me someday. But I have a question.
When we run Shapiro-Wilk test, the homogenity of variances is a
mandatory condition?
No it is not. An homoscedasticity test only makes sense when you have a
grouping factor, and a normality test may of course be used in a variety of
contexts when you have a unique sample.
My point was: If you use gaussian models and assume homogeneity of
within-group variances, then testing normality is somewhat simplified, since
your model residuals are expected to be drawn from a unique normal
distribution, and only one normality test on the residuals is necessary (no
need for a loop).
Or, as some of us would say, *no* normality test on the residuals is necessary,
which is even simpler.
-thomas
Thomas Lumley Assoc. Professor, Biostatistics
[email protected] University of Washington, Seattle
______________________________________________
[email protected] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
