[Rd] sd(NA)

[Prof Brian Ripley]

On Sun, 2 Dec 2007, Wolfgang Huber wrote:

> Dear Prof. Ripley
>
> I noted a change in the behaviour of "cov", which is very reasonable:
>
> ## R version 2.7.0 Under development (unstable) (2007-11-30 r43565)
> >  cov(as.numeric(NA), as.numeric(NA), use="complete.obs")
> Error in cov(as.numeric(NA), as.numeric(NA), use = "complete.obs") :
>   no complete element pairs
>
> whereas earlier behavior was, for example:
> ## R version 2.6.0 Patched (2007-10-23 r43258)
> > cov(as.numeric(NA), as.numeric(NA), use="complete.obs")
> [1] NA
>
>
> I wanted to ask whether the effect this has on "sd" is desired:
>
> ## R version 2.7.0 Under development (unstable) (2007-11-30 r43565)
> > sd(NA, na.rm=TRUE)
> Error in var(x, na.rm = na.rm) : no complete element pairs
>
> ## R version 2.6.0 Patched (2007-10-23 r43258)
> >  sd(NA, na.rm=TRUE)
> [1] NA

That is a bug fix: see the NEWS entry.  The previous behaviour of

> sd(numeric(0))
Error in var(x, na.rm = na.rm) : 'x' is empty
> sd(NA_real_, na.rm=TRUE)
[1] NA

was not as documented:

      This function computes the standard deviation of the values in
      'x'. If 'na.rm' is 'TRUE' then missing values are removed before
      computation proceeds.

so somehow an empty vector had a sd() if computed one way, and not if 
computed another.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595