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st: Re: Normality Testing


From   "Karamjit Shad" <karamjit_shad@hotmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Re: Normality Testing
Date   Wed, 11 Feb 2004 09:05:24 +0000

Prior to carrying out an anova I tested my data for normality and some of the data was non-normal. Ladder suggested a log transformation would be suitable. I then checked the transformed data using swilk and the data is still non-normal. However sfrancia indicates that it is normal.
. swilk igg60 if group==3

Shapiro-Wilk W test for normal data
Variable | Obs W V z Prob>z
-------------+-------------------------------------------------
igg60 | 30 0.74827 8.001 4.300 0.00001

. swilk ligg60 if group==3

Shapiro-Wilk W test for normal data
Variable | Obs W V z Prob>z
-------------+-------------------------------------------------
ligg60 | 30 0.91745 2.624 1.995 0.02305

. sfrancia ligg60 if group==3

Shapiro-Francia W' test for normal data
Variable | Obs W' V' z Prob>z
-------------+-------------------------------------------------
ligg60 | 30 0.93170 2.398 1.600 0.05479

a qnorm plot shows the data to "gently" oscillate about the normal distribution but nothing that would worry me too much.
My question is what test should I use for testing for normality in this situation - or should I just use a non-parametric analysis.
Thank you
Karamjit

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