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

From: <goodrich_at_fas.harvard.edu>
Date: Thu 21 Dec 2006 - 01:08:53 GMT


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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.

Thanks,
Ben

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