Stratum sample sizes aren't maintained using rsample bootstraps() function.

[JVAdams]

When taking stratified bootstrap samples from a data frame, I expected separate bootstrap samples to be taken within each stratum, so that the resulting bootstrap sample has the same number of observations in each stratum as the original data frame. However, that is not the case when using the bootstraps() function of the rsample package. When I run this code:

library(rsample)

mydf <- data.frame(A=1:58, B=rep(1:4, c(6, 6, 23, 23)))
lboots <- bootstraps(mydf, times=3, strata="B")$splits
lbootsdf <- lapply(lboots, as.data.frame)

with(mydf, table(B))
lapply(lbootsdf, function(df) table(df$B))

These are the results I get:

B
 1  2  3  4 
 6  6 23 23 


$`1`
 1  2  3  4 
10  5 20 23 

$`2`
 1  2  3  4 
 3  8 24 23 

$`3`
 1  2  3  4 
 4  5 24 25 

I was expecting to see 6 1's, 6 2's, 23 3's, and 23 4's in each of the three bootstrap samples.

When I posted a query on stackoverflow, joran commented that the function make_strata by default pools strata below 15% of the total, with no way to adjust that parameter from the calling functions, like boostraps(). This pooling is not mentioned in the help documentation for the bootstraps() function.

sessionInfo()
R version 3.5.2 (2018-12-20)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows >= 8 x64 (build 9200)

Matrix products: default

locale:
[1] LC_COLLATE=English_United States.1252 
[2] LC_CTYPE=English_United States.1252   
[3] LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C                          
[5] LC_TIME=English_United States.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets 
[6] methods   base     

other attached packages:
[1] rsample_0.0.4 tidyr_0.8.2  

loaded via a namespace (and not attached):
 [1] Rcpp_1.0.0       crayon_1.3.4     dplyr_0.8.0.1   
 [4] assertthat_0.2.0 R6_2.4.0         magrittr_1.5    
 [7] pillar_1.3.1     rlang_0.3.1      rstudioapi_0.9.0
[10] generics_0.0.2   tools_3.5.2      glue_1.3.0      
[13] purrr_0.3.0      yaml_2.2.0       compiler_3.5.2  
[16] pkgconfig_2.0.2  tidyselect_0.2.5 tibble_2.0.1    

[sxmorgan]

I don't have a solution, but came here hoping for one because I am having the exact same issue... did you have any luck or come up with a workaround?

[JVAdams]

See the answers to my stackoverflow query.

[juliasilge]

The PR in #149 lowers the threshold for strata pooling to 10% of the total and adds documentation to each function so that users can be more clear on what's going on with their groups!

[github-actions]

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