On Sat, 26 Jun 2010, Bryan Hanson wrote:

Thanks Chuck, I understand much better what is going on with your example.
But I'm still uncertain why the b2$t array does not have the dimensions of R
x no. of strata.

Because the test statistic returned by mm() is a scalar. It has nothing to do with the use or number of strata.

Look at what the first case in example( boot ) is doing:

ncol(boot(grav1, diff.means, R=999, stype="f")$t)
[1] 2
ncol(boot(grav1, diff.means, R=999, stype="f",strata=grav1[,1])$t)
[1] 2
diff.means(grav1,1:nrow(grav1))
[1] -4.100549 14.722902


Chuck


Any further insight would be appreciated.  Bryan
*************
Bryan Hanson
Acting Chair
Professor of Chemistry & Biochemistry
DePauw University, Greencastle IN USA



On 6/26/10 12:43 PM, "Charles C. Berry" <cbe...@tajo.ucsd.edu> wrote:

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@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:cbe...@tajo.ucsd.edu             UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901



______________________________________________
R-help@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:cbe...@tajo.ucsd.edu               UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901

______________________________________________
R-help@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.

Reply via email to