From ripley at stats.ox.ac.uk  Mon Apr 11 16:57:32 2011
From: ripley at stats.ox.ac.uk (Prof Brian Ripley)
Date: Mon, 11 Apr 2011 15:57:32 +0100
Subject: [Rd] Inconsistency between rowMeans documentation and reality?
In-Reply-To: <1302003186.2870.7.camel@chrysothemis.geog.ucl.ac.uk>
References: <1302003186.2870.7.camel@chrysothemis.geog.ucl.ac.uk>
Message-ID: <alpine.LFD.2.02.1104111553570.32033@gannet.stats.ox.ac.uk>

I suspect you omitted some of the help page:

   As they are written for speed, they blur over some of the subtleties
   of ?NaN? and ?NA?.

So, given that (and that real NA is a specific NaN) I think it is 
perfectly reasonable to claim they are consistent with mean.

On Tue, 5 Apr 2011, Gavin Simpson wrote:

> Dear List,
>
> I'm not even sure this is an issue or not, but ?rowMeans has:
>
> Value:
>
>     A numeric or complex array of suitable size, or a vector if the
>     result is one-dimensional.  The ?dimnames? (or ?names? for a
>     vector result) are taken from the original array.
>
>     If there are no values in a range to be summed over (after
>     removing missing values with ?na.rm = TRUE?), that component of
>     the output is set to ?0? (?*Sums?) or ?NA? (?*Means?), consistent
>     with ?sum? and ?mean?.
>
> However the output of mean() and rowMeans() is not exactly the same when
> all supplied values are missing.
>
>> mean(NA, na.rm = TRUE)
> [1] NaN
>> mean(rep(NA, 5), na.rm = TRUE)
> [1] NaN
>> rowMeans(matrix(rep(NA, 5), ncol = 5), na.rm = TRUE)
> [1] NA
>
> So in one sense, the outputs are not consistent:
>
>> is.nan(mean(rep(NA, 5), na.rm = TRUE))
> [1] TRUE
>> is.nan(rowMeans(matrix(rep(NA, 5), ncol = 5), na.rm = TRUE))
> [1] FALSE
>
> but in another they are:
>
>> is.na(mean(rep(NA, 5), na.rm = TRUE))
> [1] TRUE
>> is.na(rowMeans(matrix(rep(NA, 5), ncol = 5), na.rm = TRUE))
> [1] TRUE
>
> I'm not familiar enough with the details to know if this even matters,
> but wonder if something in the documentation needs a change or tweak to
> clarify what is returned. As I say, in one sense the outputs are not
> consistent.
>
>> sessionInfo()
> R version 2.13.0 beta (2011-04-04 r55298)
> Platform: x86_64-unknown-linux-gnu (64-bit)
>
> locale:
> [1] LC_CTYPE=en_GB.utf8       LC_NUMERIC=C
> [3] LC_TIME=en_GB.utf8        LC_COLLATE=en_GB.utf8
> [5] LC_MONETARY=C             LC_MESSAGES=en_GB.utf8
> [7] LC_PAPER=en_GB.utf8       LC_NAME=C
> [9] LC_ADDRESS=C              LC_TELEPHONE=C
> [11] LC_MEASUREMENT=en_GB.utf8 LC_IDENTIFICATION=C
>
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods
> [7] base
>
> loaded via a namespace (and not attached):
> [1] tools_2.13.0
>
> Thanks,
>
> Gavin
> -- 
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> Dr. Gavin Simpson             [t] +44 (0)20 7679 0522
> ECRC, UCL Geography,          [f] +44 (0)20 7679 0565
> Pearson Building,             [e] gavin.simpsonATNOSPAMucl.ac.uk
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>
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>

-- 
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


[Rd] Inconsistency between rowMeans documentation and reality?
Prof Brian Ripley