[R] Is this a bug (or a feature) in hist(x)$density ??

[David Winsemius]

On Mar 13, 2010, at 3:39 PM, Tal Galili wrote:

> Hi all,
>
> A friend send me a question on why does this:
>
> x<-rpois(100,1)
> sum( hist(x)$density )
>
> Gives out "2"

Yes. And...
 > hist(x)$breaks
[1] 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

>
> I tried this:
>
> sum( hist(x, freq =T)$density )
>
> It didn't help.
>
> Then he came back with the following insight:
>
> # with breaks
> b<-c(0,0.9,1:8)
> sum(hist(x,breaks=b)$density) # Much more then 2
> # but if we add weights according to the interval length
> sum(hist(x,breaks=b)$density * diff(b))
> # it works
>
>
>
> What do you think ?
>

I do think that neither you nor your friend understand densities.

-- 
David.

David Winsemius, MD
West Hartford, CT