# [Rd] meaning of "trim" in mean()

Prof Brian Ripley ripley at stats.ox.ac.uk
Sun Oct 28 07:54:58 CET 2007

```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.
>
>

--
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

```