[R] sum of deviations from the weighted mean

[R. Michael Weylandt <michael.weylandt@gmail.com>]

R FAQ 7.31 I'd imagine. (Floating point arithmetic and limitations thereof)

Michael

On Dec 8, 2011, at 9:15 AM, andrija djurovic <djandrija at gmail.com> wrote:

> Hi all. I tried to calculate sum of deviations from the weighted mean and i
> didn't get what i expected - 0. Here is an example:
> 
>> wt <- c(10,25,38,22,5)
>> x <- 6:10
>> wm <- weighted.mean(x,wt)
>> (x-wm)*wt
> [1] -18.70 -21.75   4.94  24.86  10.65
>> sum((x-wm)*wt)
> [1] -1.24345e-14
> 
> With simple mean I got 0:
>> sum(x-mean(x))
> [1] 0
> 
> Could someone explain me why we didn't get 0, with weighted mean, as it is
> expected?
> Thanks in advance
> 
> Andrija
> 
>    [[alternative HTML version deleted]]
> 
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