# [R] trimmed mean in R seems to round the trimming fraction

Sundar Dorai-Raj sundar.dorai-raj at pdf.com
Sun Sep 18 22:11:11 CEST 2005

```
VINOD at FORDHAM.EDU wrote:
>
>
> subject: trimmed mean in R seems to round the trimming fraction
> to r-help at stat.math.ethz.ch.
>
> Consider the following example of 10 numbers.  10% trimmed mean is correct
> but you can see that the result is the same for many trimming fractions
> till 0.20!
>
> For example 13% trimmed mean should use interpolation of second and
> eighth ordered observation.  R does not seem to do this.
> The correct 13% trimmed mean is the average of 7.4 middle observations
> (28+46+50+52+54+63+82+64.4)/7.4
>
>
>
>>z=c(23, 40,     46,      50,      52,      54,    63,    82,92,98)
>>mean(z, trim=0.10)
>
>  59.875
>
>>mean(z, trim=0.13) # wrong result is  59.875
>
>  59.875
>
>>mean(z, trim=0.15)
>
>  59.875
>
>>mean(z, trim=0.18)
>
>  59.875
>
>>mean(z, trim=0.1999999)
>
>  59.875
>
>>mean(z, trim=0.20)
>
>  57.83333
>
>>mean(z, trim=0.40)
>
>  53
>
>

Hi,

The "trim" argument works as documented. Did you read ?mean:

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.

Also, I don't recall ever learning your method for trimmed mean, but R
is a programming language and you can always write your own trimmed mean
function.

HTH,

--sundar

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