On Aug 17, 2010, at 4:57 PM, maiya wrote:
Hi! This is going to be a real newbie question, but I can't figure
it out.
I'm trying to plot densities of various functions of chi-square. A
simple
chi-square plot I can do with dchisq(). But e.g. chi.sq/degrees of
freedom I
only know how to do using density(rchisq()/df). For example:
plot(1, type="n", xlab="", ylab="", xlim=c(0,2), ylim=c(0,7))
for (i in c(10,50,100,200,500)){
lines(density(rchisq(100000,i)/i))
}
But even with 100,000 samples the curves still aren't smooth. Surely
there
must be a more elegant way to do this?
If you want smooth curves then dchisq is the way to go. My guess is
that you have not yet figured out that there is a second parameter
which controls the df of the dchisq output (as well as having a third
non-centrality parameter):
set.seed(123)
x <- rchisq(100, 5)
hist(x, prob=TRUE)
curve( dchisq(x, df=5), col='red', add=TRUE)
Thanks!
Maja
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