st: Re: vanelteren

 From Ricardo Ovaldia <[email protected]> To [email protected] Subject st: Re: vanelteren Date Fri, 29 Jul 2005 07:36:38 -0700 (PDT)

```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.
> treatment clinic- (or perhaps -xtmixed response
> treatment || clinic:-), you'd
> have -vanelteren response, by(treatment)
> strata(clinic)-.
>
> Joseph Coveney
>
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>

Ricardo Ovaldia, MS
Statistician
Oklahoma City, OK

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