[Rd] Quantile issue

Martin Maechler maechler at stat.math.ethz.ch
Thu May 1 22:20:56 CEST 2014

>>>>> Therneau, Terry M , Ph D <therneau at mayo.edu>
>>>>>     on Wed, 30 Apr 2014 12:00:33 -0500 writes:

    > This is likely yet another instance of round off error,
    > but it caught me by surprise.  tmt% R --vanilla (headers
    > skipped, version 3.0.2 on Linux)

    >> load('qtest.rda') length(temp)
    > [1] 3622
    >> max(temp) >= quantile(temp, .98)
    >    98% FALSE

    > I can send the file to anyone who would like to understand
    > this more deeply.  The top 3% of the vector is a single
    > repeated value.

Hmm... this may be the same as PR#15746 :


on which I commented that it was an example of  FAQ 7.31 :

  > Martin Maechler 2014-04-10 07:10:41 UTC

  > What really happens is simply

  > > x0 <- 0.000384478262997113
  > > h <- (1:9)/10
  > > diff(range((1-h)*x0 + h*x0))
  > [1] 1.084202e-19
  > > 

  > and that of course *is* just  R FAQ 7.31

  > I'm not sure this is something we want to amend;  we *could*
  > ensure

  > 	   min(x) <= quantile(x, *) <= max(x)

  > with a bit of extra code that makes *all* calls of quantile
  > slower than
  > now... just for the sake of this one very very rare exception ?

  > Alternatively we just document it .. 
  > Maybe this needs public discussion on R-devel 

and so I did bring it to R-devel.
If look at the fuller report, you also see that I claim that
this seems to happen only for very rare numbers...,
I had a loop running, searching for such numbers as the above x0
and it did not find one in many hours...,
OTOH, you now found a second example (of the same phenomenon I think).

So, yes, please, send me the qtest.rda...
or to all of us a dput(.) of the top 5% of the data which may
suffice for a reproducible example.

Also, it seems that for these rare examples we may need to rewrite
the code of quantile.default() ... making it less nice, less
transparent, not something I think we should commit lightly.


    > Terry Therneau

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