Ricardo Ovaldia wrote:
I am a bit concerned about using -vanelteren- on these data. My understanding
is that this test performs a rank-sum test for each dam and then constructs a
weighted linear combination of these Wilcoxon statistics. My problem is that
the rank-sum
test assumes that you have two independent samples, however in my case, the
embryos allocated to the two medias come from the same dam and thus are not
independent within strata (dam).
Is the test still valid?
--------------------------------------------------------------------------------
You can think of van Elteren's test as analogous to an analysis of variance
with a blocking factor. In the normal case, you would have -anova embryo_score
culture_medium dam-. Dam is a blocking factor within levels of which you
anticipate clustering of scores. Conditional on dam, however, the scores would
be independent.
One common use of the van Elteren test is with clinic as the stratification
variable in a multicenter clinical study. van Elteren's test is used in lieu
of randomized blocks ANOVA with clinics as blocks. Instead of -anova response
treatment clinic- (or perhaps -xtmixed response treatment || clinic:-), you'd
have -vanelteren response, by(treatment) strata(clinic)-.
Joseph Coveney
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
