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
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Charles C. Berry (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:cbe...@tajo.ucsd.edu UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901
______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
