[R] quantile() depends on order of probs?

[Joshua Wiley]

Hello All,

I am trying to figure out the rational behind why quantile() returns
different values for the same probabilities depending on whether 0 is
first.

Here is an example:

quantile(c(54, 72, 83, 112), type=6, probs=c(0, .25, .5, .75, 1))
quantile(c(54, 72, 83, 112), type=6, probs=c(.25, .5, .75, 1, 0))

It seems to come down to this part of the code for quantile:

fuzz <- 4 * .Machine$double.eps
nppm <- a + probs * (n + 1 - a - b)
j <- floor(nppm + fuzz)
h <- nppm - j
qs <- x[j + 2L]
qs[h == 1] <- x[j + 3L][h == 1]
other <- (h > 0) && (h < 1)
if (any(other))
  qs[other] <- ((1 - h) * x[j + 2L] + h * x[j + 3L])[other]

In my example, a and b are both 0, and n = 4.  Particularly, the
alternate formula for qs is only used when the first element of h is
both > 0 and < 1.  Any ideas on this?  It seems like a simple
alternative would be

other <- (h > 0) & (h < 1)

but I do not know if that would cause problems for other quantile
formulae.  By the way, this comes around lines 39-70 in
quantile.default in:

> version
               _
platform       x86_64-pc-mingw32
arch           x86_64
os             mingw32
system         x86_64, mingw32
status
major          2
minor          11.1
year           2010
month          05
day            31
svn rev        52157
language       R
version.string R version 2.11.1 (2010-05-31)


Best regards,

Josh

-- 
Joshua Wiley
Ph.D. Student
Health Psychology
University of California, Los Angeles