Thanks David and Stephan, David's reply was very helpful since I now realize how I made in my head the (wrong) assumption that the hist would automatically produce rectangular bars with the width of 1 (since the distribution is a discrete one). The call: hist(x)$breaks Helped to bring me back to focus.
Stephan, I remember studying about "Riemann integral" but didn't remember that's how it is called - thanks for the link. Tal ----------------Contact Details:------------------------------------------------------- Contact me: tal.gal...@gmail.com | 972-52-7275845 Read me: www.talgalili.com (Hebrew) | www.biostatistics.co.il (Hebrew) | www.r-statistics.com (English) ---------------------------------------------------------------------------------------------- On Sat, Mar 13, 2010 at 11:04 PM, Stephan Kolassa <stephan.kola...@gmx.de>wrote: > Hi Tal, > > basically, by summing over the (pointwise) density, you are "approximating" > the integral over the density (which should be around 1) - but to really do > a rectangular approximation, you will of course need to multiply each > function value by the width of the corresponding rectangle. I'd recommend > reading through Wikipedia's article on the Riemann integral, you may be > enlightened. > http://en.wikipedia.org/wiki/Riemann_integral > > HTH, > Stephan > > > Tal Galili schrieb: > >> Hi all, >> >> A friend send me a question on why does this: >> >> x<-rpois(100,1) >> sum( hist(x)$density ) >> >> Gives out "2" >> >> 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 ? >> >> >> Tal >> >> >> >> ----------------Contact >> Details:------------------------------------------------------- >> Contact me: tal.gal...@gmail.com | 972-52-7275845 >> Read me: www.talgalili.com (Hebrew) | www.biostatistics.co.il (Hebrew) | >> www.r-statistics.com (English) >> >> ---------------------------------------------------------------------------------------------- >> >> [[alternative HTML version deleted]] >> >> ______________________________________________ >> R-help@r-project.org mailing list >> https://stat.ethz.ch/mailman/listinfo/r-help >> PLEASE do read the posting guide >> http://www.R-project.org/posting-guide.html >> and provide commented, minimal, self-contained, reproducible code. >> >> > [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
