[R] how to overlay the histogram with fitted gamma density plot (emergent!!)

Peter Dalgaard BSA p.dalgaard at biostat.ku.dk
Wed Oct 16 23:28:24 CEST 2002

xiaodong jin <jin_xiaodong at yahoo.com> writes:

> For a real data column X value ranged between (56.4521,32317.9) with missing values, I need to overlay 2 plots: histogram & fitted gamma density.
> I use following to generate histogram.
> xbk_seq(50,33000,by=100)
> hist(x,breaks=xbk)
> But I don't know how to get "fitted gamma density"?
> In SAS proc capability, I got Shape=2.59, Scale=3481).
> But when I do
> plot(dgamma(x, shape=2.59, rate=1,scale=3481)), it gives me a flat densed line close to 0, which does not look correct.
> Would anybody help me on this?

First of all, you need to make sure that your histogram is on "density
scale" (i.e. not raw bin counts) by using prob=TRUE. Then comes the
issue of checking that SAS and R agree on the parametrization of the
gamma distribution. 

The MASS library has fitdistr() which might be helpful.

   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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