[R] bootstrapping vectors of unequal length

[Matt Oliver]

Dear R Help List,

I have a vector of n and a vector of n-1 and I want to use boot() to
bootstrap the ratio of their respective medians. I want to eventually
use boot.ci() to generate confidence intervals. Because the vectors
are not equal in length, I tried a few things, but have yet to be
successful.

Try 1: 

> x <- runif(20)
> 
> y <- c(runif(19), NA)
> 
> median(x)
[1] 0.522284
> 
> median(y[1:19])
[1] 0.488046
> 
> median(x)/median((y)[1:19])
[1] 1.070153
> 
> t <- as.data.frame(cbind(x, y))
> 
> ratio <- function(t, i) median(t$x[i])/median((t$y[1:19])[i])
> 
> boot(t, ratio, R = 1000)

ORDINARY NONPARAMETRIC BOOTSTRAP


Call:
boot(data = t, statistic = ratio, R = 1000)


Bootstrap Statistics :
    original  bias    std. error
t1*       NA      NA   0.4603294


I thought this might be successful because median(x)/median((y)[1:19])
gives a result, and not an NA.

I also tried to use a regular list (even though boot() technically
doesn't accept them) so I didn't have to use NA.

Try 2:

> x <- runif(20)
> 
> y <- runif(19)
> 
> median(x)
[1] 0.732906
> 
> median(y)
[1] 0.5596225
> 
> median(x)/median(y)
[1] 1.309644
> 
> t <- list(x = x, y = y)
> 
> ratio <- function(t, i) median(t$x[i])/median(t$y[i])
> 
> boot(t, ratio, R = 1000)

ORDINARY NONPARAMETRIC BOOTSTRAP


Call:
boot(data = t, statistic = ratio, R = 1000)


Bootstrap Statistics :
    original       bias    std. error
t1* 1.153598 -0.004907764  0.08266257


At first glance this seemed to work, but median(x)/median(y) is not
equal to the "original" in the Bootstrap Statistics (which is a bit
odd to me, but it may be because boot() doesn't accept this kind of
list.)


Is there a way to do this type of bootstrap with the boot() function?

Thanks in advance

Matt Oliver