Thanks Chuck, I understand much better what is going on with your example. But I'm still uncertain why the b2$t array does not have the dimensions of R x no. of strata.
Any further insight would be appreciated. Bryan ************* Bryan Hanson Acting Chair Professor of Chemistry & Biochemistry DePauw University, Greencastle IN USA On 6/26/10 12:43 PM, "Charles C. Berry" <cbe...@tajo.ucsd.edu> wrote: > On Sat, 26 Jun 2010, Bryan Hanson wrote: > >> Hello All. I must be missing the really obvious here: >> >> mm <- function(d, i) median(d[i]) >> b1 <- boot(gravity$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.