Re: st: Normal Distribution

[Arman Eshaghi <armaneshaghi@aol.com>]

Thanks for the clear response Maarten.


-----Original Message-----
From: Maarten Buis <maartenlbuis@gmail.com>
To: statalist <statalist@hsphsun2.harvard.edu>
Sent: Wed, Sep 14, 2011 1:16 pm
Subject: Re: st: Normal Distribution


On Wed, Sep 14, 2011 at 12:39 PM, Arman Eshaghi wrote:> I have a 
question concerning normal distribution and normal plots. I have a> 
sample of 250 subjects with some psychological scores that are not 
normally> distributed. With scaled score conversion and cumulative 
distribution> function I tried to approximate the distribution to 
normal distribution.> However, kolmogorov-smirnov test is still 
significant which refutes the normality.> Q-Q plots on the other hand 
show promising results for scaled scores. Does> my data considered 
normally distributed in this case? My goal is to use> these scores in 
discriminant analysis which presumes the data has a> multivariate 
normal distribution.The Kolmogorov-Smirnov test is particularly 
sensitive to deviations inthe middle of the distribution, while 
QQ-plots (-qnorm-) areparticularly sensitive to what happens in the 
tails. So, it is nosurprise that these two approaches lead to different 
conclusions.Apparently there is something interesting going on in the 
center ofthe distribution, which you can better see with a PP-plot 
(-pnorm-).Anyhow, what you are looking at is the marginal distribution, 
and thatdoes _not_ have to look anything like a normal distribution. So 
foryour aim these tests and graphs are useless.Hope this 
helps,Maarten--------------------------Maarten L. BuisInstitut fuer 
SoziologieUniversitaet TuebingenWilhelmstrasse 3672074 
TuebingenGermanyhttp://www.maartenbuis.nl--------------------------**   
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