[R] about comparison of KURTOSIS in package: moments and fBasics
spencer.graves at pdf.com
Fri Dec 9 05:20:50 CET 2005
There are doubtless tests for kurtosos by itself, though I'm not
familiar with any. When I'm conderned about kurtosis (which is often),
I routinely make normal probability plots of observations and residuals
from model fits. If I see roughly a straight line, I conclude that I
won't likely be too misled by assuming normality. If I see a smooth "S"
shape with long tails, I would be inclined to try to fit a Student's t
model. If I see relatively sharp breaks, that's the fingerprint of a
For mixture distributions, I like Titterington, Smith, and Makov
(1986) Statistical Analysis of Finite Mixture Distributions (Wiley).
There are more recent books available, but I haven't seen a better
discussion of normal plots of mixtures. There are R packages for
estimating mixtures of various kinds. To find them, I suggest you try
"RSiteSearch" with a variety of different key phrases. For Student's t
models, I suggest you try "fitdistr" in library(MASS) or "RSiteSearch"
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hope this helps.
Baize, Harold wrote:
> Thanks to Spencer Graves for providing links to explain
> the various types of kurtosis reported by R packages.
> Spencer Graves>>(http://mathworld.wolfram.com/k-Statistic.html).
> Spencer also said:
> SG>> However, these are little used, as the estimates are known to be so
> SG>> highly variable. It is generally preferred to transform to normality or
> SG>> to use some other distribution and then use maximum likelihood.
> This advice is good if your interest is comparing models, but what
> if variation in kurtosis itself is your interest? I am wondering if
> someone could provide some direction for answering questions about
> differences between samples in kurtosis. There are tests of
> significance for means and variance. How would one test hypotheses
> of difference in kurtosis between samples?
> Thanks in advance.
> Harold Baize
> Youth Services Evaluator
> Butte County Department of Behavioral Health
> R-help at stat.math.ethz.ch mailing list
> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Spencer Graves, PhD
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