Thank you Joseph.
Yes, you are correct; conditional on the dam the
scores would be independent. My doubt arose because I
get a highly statistical difference where I was not
expecting one. The mean from the two culture mediums
are similar (9.2 vs. 10.2) and the medians and rage
are exactly the same:
N Mean (SD) Median Min - Max
-----------------------------------------------------
Percent_0 7010 9.2 (8.2) 10.0 0.0 - 50.0
Percent_1 7390 10.2 (9.2) 10.0 0.0 - 50.0
-----------------------------------------------------
However, -vanelteren- yields p=0.0054 (asymptotic)
p=0.002 (permutation based) and that seems strange:
. vanelteren percent, by(mm) strata(dam)
Generalized Wilcoxon-Mann-Whitney Ranksum Test (van
Elteren's Test)
Variance of
Weighted Expected
Weighted
Stratum | n | Ranksum | Ranksum |
Ranksum
-------------+-------+-----------+----------+-------------
1 | 6 | 1.00 | 1.0 | 0.068
2 | 2 | 0.50 | 0.5 | 0.000
3 | 4 | 1.00 | 1.0 | 0.000
4 | 3 | 1.00 | 1.0 | 0.042
.
.
.
234 | 6 | 1.57 | 1.0 | 0.087
235 | 9 | 1.40 | 2.0 | 0.113
236 | 6 | 0.86 | 1.0 | 0.084
-------------+-------+-----------+----------+-------------
Sums |1,485 | 375.02 | 363.0 |
18.646
Asymptotic test statistic
z = 2.784
Prob(Z > |z|) = .0054
Regards,
Ricardo
--- Joseph Coveney <[email protected]> wrote:
> 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:
> *
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>
Ricardo Ovaldia, MS
Statistician
Oklahoma City, OK
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