Re: [Rd] meaning of "trim" in mean()

From: Peter Dalgaard <p.dalgaard_at_biostat.ku.dk>
Date: Sun, 28 Oct 2007 14:42:49 +0100

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.
>>>
>>>
>>>
>>
>>
>>

-- 
   O__  ---- Peter Dalgaard             ุster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard_at_biostat.ku.dk)                  FAX: (+45) 35327907

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