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st: re: conflicting tests for normality
From
Kouji Asakura <[email protected]>
To
[email protected]
Subject
st: re: conflicting tests for normality
Date
Fri, 25 Feb 2011 05:48:46 -0800 (PST)
Hi Statalisters,
I need help with a problem I'm having. I'm testing for normality of a variable
and I made use of the tests in Stata; Shapiro-Wilk, the sktest, and
Shapiro-Francia. However, I obtained conflicting results.
. sktest black
Skewness/Kurtosis tests for Normality
------- joint ------
Variable | Obs Pr(Skewness) Pr(Kurtosis) adj chi2(2) Prob>chi2
-------------+---------------------------------------------------------------
black | 82 0.926 0.003 7.76
0.0207
. sktest black, noadjust
Skewness/Kurtosis tests for Normality
------- joint ------
Variable | Obs Pr(Skewness) Pr(Kurtosis) chi2(2) Prob>chi2
-------------+---------------------------------------------------------------
black | 82 0.926 0.003 8.69 0.0130
.swilk black
Shapiro-Wilk W test for normal data
Variable | Obs W V z Prob>z
-------------+--------------------------------------------------
black | 82 0.97105 2.028 1.551 0.06043
. sfrancia black
Shapiro-Francia W' test for normal data
Variable | Obs W' V' z Prob>z
-------------+--------------------------------------------------
black | 82 0.97866 1.640 0.986 0.16212
So you see, the -sktest- says it's not normal, while both Shapiro tests say the
opposite, at least at a 0.05 alpha.
Which result should I follow? Would really appreciate your expertise in the
matter.
Thanks!
Kōji Asakura
奈良大学
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