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
Gower Street, London [w] http://www.ucl.ac.uk/~ucfagls/
UK. WC1E 6BT. [w] http://www.freshwaters.org.uk
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Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
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