On Sun, 16 Jan 2011, Hugo Mildenberger wrote:
Using R-2.12.1 and latticeExtra-0.6-14, I would like to understand
why a rootogram displaying samples from the Poisson distribution looks like I
expected it, whereas a rootogram using the normal distribution does not:
library(latticeExtra)
rootogram(~rpois(1000, lambda = 50), dfun = function(x) dpois(x, lambda = 50))
rootogram(~rnorm(1000), dfun = function(x) dnorm(x,mean(x),sd(x)))
I probably can't attach figures here. Thus a textual description of what
I get may suffice: With increasing sample size, the rootogram using
random samples from the Poisson distribution shows decreasing
differences (bars are quickly approaching the zero line), whereas the
displayed differences for random samples of the normal distribution are
always large. The differences even increase with sample size, i.e, the
hanging bars tend to vanish for very large samples.
The normal distribution is a continuous distribution, i.e., the frequency
for each observed value will essentially be 1/n and not converge to the
density function. Hence, you would need to look at histogram or smoothed
densities. Rootograms, on the other hand, are intended for discrete
distributions.
Z
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