[R] Re: R: log-normal distribution and shapiro test

Siegfried Gonzi siegfried.gonzi at stud.uni-graz.at
Wed Nov 17 15:01:37 CET 2004


Thanks for your answer.

Do you know how to test whether the data would fit to a gamma-distribution?

How can I call fBasics?

Note: I installed R-language on my Macintosh today; I have used the 
binary -- pre compiled -- package.

Some of the R-help facilties do not function on my Mac.

Again to my data: How can I compute the skew? I think I lack some basic 
packages - right?

The curious things actually is that the median and the mean are quite 
similar, e.g. 0.19 and 0.2 respectively; the skew is about 1.0 (I 
calculated the skew by my own computer code in Bigloo).

The problem actually is: my boss expects from me that I make some tests; 
personally I am a bit generous and everything is a Gaussian or 
log-Gaussian distribution, because how can I be sure that the underlying 
data to not have any serious flaws? Statistics is black art - right?

S. Gonzi

Vito Ricci wrote:

>from what you're writing:
>"The logaritmic transformation
>"shapiro.test(log10(y))" says: W=0.9773, p-value=
>2.512e-05." it seems the log-values are not
>distributed normally and so original data are not
>distributed like a log-normal: the p-value is
>extremally small!
>Other tests for normality are available in package:
>compare the log-transformation of your ecdf with
>normal cdf: see ? ecdf
>use qqnorm and qqplot
>did you calculate skewness and kurtosis? see in
>package fBasics.
>I remember to you that the log-normal distribution as
>three parameters: shape parameter, location parameter
>and scale parameter. Transfroming by the simple log,
>you are missing the location parameter, or implicitely
> you assuming is =0.
>for more news about log-normal distribution.
>I hope I give you a little help.
>you wrote:
>Yes I know that sort of questions comes up quite
>often. But with all due 
>respect I din't find how to perform what I want. I am
>searching archives 
>and bowsing manuals but it isn't there, though, it is
>a ridiculous 
>simple task for the experienced R user.
>I have data and can do the following with them:
>hist(y, prob=TRUE)
>The result actually is a nice histogram superimposed
>by a line plot.
>The histogram is a bit skewed to the left. My
>assumption actually is 
>that a log-normal transformation would cure the
>problem. But how the 
>hell can one plot such a density function or Gaussian
>function which has 
>logarithmic scales on x axis.
>For example I tried:
>But with no avail. I want  my axis like: 1,10,100
>What would be other methods to test whether the data
>are logaritmically 
>A last question to the Shapiro-Wilk test. Were can I
>get critical 
>parameters? I mean I get for my distribution:
>W=0.9686, p-value=6.887e-07.
>What does that mean? Yes I have got some books about
>statics, but none 
>of them says what one should do with the values then.
>The logaritmic 
>transformation "shapiro.test(log10(y))" says:
>W=0.9773, p-value= 2.512e-05.
>Sorry for disturbing you. Although, it is really no
>homework. I need it 
>for my Phd in physics; after a lengthy computation on
>the computer I 
>would like to go to see whether the outputs are
>log-normal or normal 
>Siegfried Gonzi
>University of Graz
>Institute for Physics
>Tel.: ++43-316-380-8620
>Diventare costruttori di soluzioni
>Became solutions' constructors
>"The business of the statistician is to catalyze 
>the scientific learning process."  
>George E. P. Box
>Visitate il portale http://www.modugno.it/
>e in particolare la sezione su Palese http://www.modugno.it/archivio/cat_palese.shtml

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