From: Charles C. Berry <cberry_at_tajo.ucsd.edu>

Date: Sat, 26 Jun 2010 09:43:40 -0700

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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. Received on Sat 26 Jun 2010 - 16:47:36 GMT

Date: Sat, 26 Jun 2010 09:43:40 -0700

On Sat, 26 Jun 2010, Bryan Hanson wrote:

> Hello All. I must be missing the really obvious here:

*>
**> mm <- function(d, i) median(d[i])
**> b1 <- boot(gravity$g, mm, R = 1000)
**> b1
**> b2 <- boot(gravity$g, mm, R = 1000, strata = gravity$series)
**> b2
**>
**> Both b1 and b2 seem to have done (almost) the same thing, but it looks like
**> the strata argument in b2 has been ignored. However, str(b1) vs str(b2)
**> does show that the strata have been noted correctly. But b2$t is a 1000 x 1
**> array, not a 1000 x 8 array (gravity$series is a factor with 8 levels).
**>
**> There is a more complex example in ?boot using the same data set that gives
**> a result that seems to make sense (2 levels in the factor, so $t has 2
**> columns).
**>
**> I either misunderstand the expected behavior or I've missed some punctuation
**> or syntax detail.
*

Your punctuation and syntax is OK.

Note:

*> SISWR <- function(x) sample(x,length(x),repl=TRUE)
**> # no strata
**> var(replicate(1000,median(SISWR(gravity$g))))
*

[1] 0.4588338

*> # now stratify on series
**> gsplit <- split(gravity$g,gravity$series)
**> var(replicate(1000,median(unlist(lapply(gsplit,SISWR)))))
*

[1] 0.3882272

*>
**> sqrt(.45) # this agrees with b1
*

[1] 0.6708204

*> sqrt(.39) # this agrees with b2
*

[1] 0.6244998

*>
*

The effect of stratification depends on the relative amount of variation within vs between strata. This suggests there is not a lot:

*> aov(g~series,gravity)
*

Call:

aov(formula = g ~ series, data = gravity)

Terms:

series Residuals Sum of Squares 2818.624 8239.376 Deg. of Freedom 7 73

Residual standard error: 10.62394

Estimated effects may be unbalanced

*>
*

**HTH,
**
Chuck

*>
*

> TIA, Bryan

*>
**> *************
**> Bryan Hanson
**> Acting Chair
**> Professor of Chemistry & Biochemistry
**> DePauw University, Greencastle IN USA
**>
**>> sessionInfo()
**> R version 2.11.0 (2010-04-22)
**> x86_64-apple-darwin9.8.0
**>
**> locale:
**> [1] en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8
**>
**> attached base packages:
**> [1] datasets tools grid graphics grDevices utils stats
**> [8] methods base
**>
**> other attached packages:
**> [1] boot_1.2-42 brew_1.0-3 faraway_1.0.4
**> [4] GGally_0.2 xtable_1.5-6 mvbutils_2.5.1
**> [7] ggplot2_0.8.7 digest_0.4.2 reshape_0.8.3
**> [10] proto_0.3-8 ChemoSpec_1.43 R.utils_1.4.0
**> [13] R.oo_1.7.2 R.methodsS3_1.2.0 rgl_0.91
**> [16] lattice_0.18-5 mvoutlier_1.4 plyr_0.1.9
**> [19] RColorBrewer_1.0-2 chemometrics_0.8 som_0.3-5
**> [22] robustbase_0.5-0-1 rpart_3.1-46 pls_2.1-0
**> [25] pcaPP_1.8-1 mvtnorm_0.9-9 nnet_7.3-1
**> [28] mclust_3.4.4 MASS_7.3-5 lars_0.9-7
**> [31] e1071_1.5-23 class_7.3-2
**>
**> ______________________________________________
**> R-help_at_r-project.org 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.
**>
*

Charles C. Berry (858) 534-2098 Dept of Family/Preventive Medicine E mailto:cberry_at_tajo.ucsd.edu UC San Diegohttp://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901

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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. Received on Sat 26 Jun 2010 - 16:47:36 GMT

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