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.

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
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University of Maryland Center for Vascular Research
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(Fax) 410-605-7913 (Please call phone number above prior to faxing)



________________________________________
From: R-help <r-help-boun...@r-project.org> on behalf of Göran Broström 
<goran.brost...@umu.se>
Sent: Wednesday, July 14, 2021 10:46 AM
To: Duncan Murdoch; r-help@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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