[Rd] Bug in dt() function in stats - accuracy only 6 dp.

Martin Maechler m@echler @ending from @t@t@m@th@ethz@ch
Thu Jan 10 11:07:45 CET 2019

>>>>> mark chappell via R-devel 
>>>>>     on Wed, 9 Jan 2019 13:10:26 +1100 writes:

    > Hi I believe the dt() function in stats has small areas of
    > inaccuracy.

    > SmallRangeMostDetaileddf150.png shows this occurring in
    > the 10-11th decimal place.

    > MyD4 is computed using Myt_NC(), at the bottom of BugL.R -
    > my function is very slow due to using mpfr, but i believe

    > accurate to the accuracy of genhypergeo() which it uses -
    > 14-15 dp.

    > LargeRangeDiffdf150.png, which is difference between dt()
    > and my function, shows spikes across -1.5 < x < 1.5.

    > I assumed probably only a problem for large df, but
    > SmallRangeCloseZerodf20.png, shows actually more of a
    > problem with
    > df = 20.  Accuracy seems to be only to 6 decimal places.

We did not get a single piece of R code.
Maybe you've wanted to attach the 'BugL.R' R script mentioned
above, but failed to use 'text/plain'  MIME encoding?

Note: If this is about using *non*central t-distribution (as the
 above "NC" suggests), then this is *not* a bug per se,
as the help page *does* mention that the acuracy there is reduced.

Still, I have created an R bugzilla account for you to report
there, i.e.,  https://bugs.r-project.org/


    > Using R version 3.5.2.

    > Cheers

    > Mark

    > [DELETED ATTACHMENT SmallRangeMostDetaileddf150.png, PNG image]
    > [DELETED ATTACHMENT LargeRangeDiffdf150.png, PNG image]
    > [DELETED ATTACHMENT SmallRangeCloseZerodf20.png, PNG image]

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