From p.dalgaard at biostat.ku.dk  Sun Oct 28 14:42:49 2007
From: p.dalgaard at biostat.ku.dk (Peter Dalgaard)
Date: Sun, 28 Oct 2007 14:42:49 +0100
Subject: [Rd] meaning of "trim" in mean()
In-Reply-To: <47244B9A.8090108@pburns.seanet.com>
References: <39B6DDB9048D0F4DAD42CB26AAFF0AFA04D88433@usctmx1106.merck.com>	<4720993C.5030105@biostat.ku.dk>
	<Pine.LNX.4.64.0710271814060.15370@gannet.stats.ox.ac.uk>
	<47244B9A.8090108@pburns.seanet.com>
Message-ID: <472491D9.20502@biostat.ku.dk>

Patrick Burns wrote:
> If the sentence in question were amended to:
>
> Values of trim outside that range ...
>
> then I think it would rule out the misinterpretation of
> the sentence.

Yup. And I maintain that although the wording may not be technically 
ambiguous,  the whole operation is about restricting the range of x, and 
therefore is easy to interpolate "the range of x after trimming from 
each end".

>
> Pat
>
>
> Prof Brian Ripley wrote:
>
>> There is only one _range_ mentioned, (0, 0.5).  I don't see how you 
>> can construe 'that range' to be a reference to anything other than 
>> (0, 0.5).
>>
>> And why do you suppose the description for argument 'trim' is 
>> referring to 'values' of a different argument?
>>
>> It is telling you what happens for values of trim < 0 or > 0.5: that 
>> is not information that it is appropriate to excise.
>>
>>
>> On Thu, 25 Oct 2007, Peter Dalgaard wrote:
>>
>>  
>>
>>> Liaw, Andy wrote:
>>>   
>>>> (I see this in both R-patched r43124 and R-devel r43233.)
>>>> In the Argument section of ?mean:
>>>>
>>>> trim     the fraction (0 to 0.5) of observations to be trimmed from 
>>>> each
>>>> end of x before the mean is computed. Values outside that range are
>>>> taken as the nearest endpoint.
>>>>
>>>> Then in the Value section:
>>>>
>>>> If trim is non-zero, a symmetrically trimmed mean is computed with a
>>>> fraction of trim observations deleted from each end before the mean is
>>>> computed.
>>>>
>>>> The description in "trim" to me sounds like Windsorizing, rather than
>>>> trimming.  Should that be edited?
>>>>
>>>>
>>>>     
>>> I think so:
>>>
>>>   
>>>> x <- sort(rnorm(10))
>>>> mean(x,trim=.1)
>>>>     
>>> [1] -0.6387413
>>>   
>>>> mean(x[2:9])
>>>>     
>>> [1] -0.6387413
>>>   
>>>> mean(x[c(2,2:9,9)]) # Winsorizing
>>>>     
>>> [1] -0.6204222
>>>
>>> So yes, it is trimming, not Winsorizing, and the last sentence in the
>>> description of "trim" is misleading and should be, well..., trimmed.
>>>
>>>
>>>   
>>
>>  
>>


-- 
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[Rd] meaning of "trim" in mean()
Peter Dalgaard