[Rd] Bug in rt() ? (PR#9422)

goodrich at fas.harvard.edu goodrich at fas.harvard.edu
Thu Dec 21 02:08:53 CET 2006

Hash: SHA1

Prof Brian Ripley wrote:
> Why do you consider this might be a bug in R rather than in your
> expectations?  Your 'expected' is not what others have expected ....

Thank you for this response. I did consider at the time that this
unexpected result could be deliberate, but help(rt) on 2.4.1 seemed to
confirm my expectations. Martin Maechler noted on the first response to
this bug report that help(rt) was not updated to describe the new
functionality of rt().

> In 2.4.1  rt(ncp) operates continuously as ncp is varied through 0.
> That is different from saying that you want a central t (omitting ncp).

I was not expecting random draws from a central t distribution to be the
same as random draws from a limiting case of a non-central t
distribution. I was expecting that redundantly typing the default
arguments of a function would produce the same result as omitting the
default arguments. Are there other functions with this discrepancy?

In other words, I do not understand why rt() continues to default to ncp
= 0, as opposed to no default argument for ncp. It would seem that
args(rt) currently implies that the draws are to be taken from a
limiting case of non-central t distribution, when this is not accurate
if the ncp argument is omitted.


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