[R] zero weights in weighted.mean

[Göran Broström]

On 2021-07-14 19:43, Sorkin, John wrote:
> Gentlemen,
> 
> At the risk of beating a dead horse, but in he spirit of learning
> more about R, aren't the two expressions functionally the same? One
> drops values where weight is zero. The other (in the case where we
> and infinity * 0, something one would not expect to see in data) also
> drops data as in R infinity * 0 = Nan. In either case the observation
> would be dropped. I am certain I am missing something, but I don't
> know what I am missing.

Try this:

 > my <- function(x, w) sum(x * w) / sum(w)

 > x <- c(1, 1 / 0)

 > w <- 1:0
 > my(x, w)

[1] NaN

 > weighted.mean(x, w)

[1] 1


See?

Best, Göran

> 
> John
> 
> John David Sorkin M.D., Ph.D. Professor of Medicine University of
> Maryland School of Medicine Associate Director for Biostatistics and
> Informatics Baltimore VA Medical Center Geriatrics, Research,
> Education, and Clinical Center Chief, Biostatistics and Informatics 
> University of Maryland School of Medicine Division of Gerontology,
> Geriatrics and Palliative Care Senior Statistician University of
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> 
> 
> 
> ________________________________________ From: R-help
> <r-help-bounces using r-project.org> on behalf of Göran Broström
> <goran.brostrom using umu.se> Sent: Wednesday, July 14, 2021 10:46 AM To:
> Duncan Murdoch; r-help using r-project.org Subject: Re: [R] zero weights in
> weighted.mean
> 
> 
> 
> Den 2021-07-14 kl. 13:16, skrev Duncan Murdoch:
>> On 14/07/2021 6:00 a.m., Göran Broström wrote:
>>> I wonder about the last sentence in the Details section of the 
>>> documentation of 'weighted.mean':
>>> 
>>> "However, zero weights _are_ handled specially and the
>>> corresponding ‘x’ values are omitted from the sum."
>>> 
>>> The return value of weighted.mean.default is
>>> 
>>> sum((x * w)[w != 0])/sum(w)
>>> 
>>> and indeed, it looks as if zero weights are getting special
>>> treatment, but what is wrong with the alternative (equivalent?)
>>> expression
>>> 
>>> sum(x * w) / sum(w)?
>>> 
>>> Is it a good idea to remove zeros from a vector before applying
>>> 'sum' to it? I don't think so. Anyway, the sentence in the
>>> documentation seems to be uncalled for.
>> 
>> Inf*0 is not zero.  Setting weights to zero on infinite
>> observations (or NA, or NaN) will give different results in your
>> two expressions.
> 
> Thanks, agreed.
> 
> G,
> 
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