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st: SV: ranksum test in Stata 12
From
Christian Trolle <[email protected]>
To
"[email protected]" <[email protected]>
Subject
st: SV: ranksum test in Stata 12
Date
Wed, 22 Jan 2014 08:52:20 +0000
Ranksum is the non-parametric version of the unpaired ttest, so for that part yes.
However do consider scrutinizing a QQ-plot with respect to normality instead of just relying on the Shapiro-Wilk's test. Visual inspection of data is crucial.
MD
Christian Trolle
-----Oprindelig meddelelse-----
Fra: [email protected] [mailto:[email protected]] På vegne af Gwinyai Masukume
Sendt: 22. januar 2014 09:09
Til: statalist
Emne: st: ranksum test in Stata 12
Hello Stata list,
I have the variable a_score which can take the values 0, 1, 2 up to 9.
I have two groups and I want to compare if a_score is the same between the two groups. Since a_score is not normally distributed I have used a non-parametric test and the p-value shows that a_score is not significantly different between the two groups if p < 0.05 is considered significant.
. swilk a_score
Shapiro-Wilk W test for normal data
Variable | Obs W V z Prob>z
-------------+--------------------------------------------------
a_score | 4610 0.99456 13.698 6.850 0.00000
.
. * non-parametric test
. ranksum a_score, by(group)
Two-sample Wilcoxon rank-sum (Mann-Whitney) test
group | obs rank sum expected
-------------+---------------------------------
Group 1 | 4504 10338974 10352444
Group 2 | 92 224932.5 211462
-------------+---------------------------------
combined | 4596 10563906 10563906
unadjusted variance 1.587e+08
adjustment for ties -2084329.2
----------
adjusted variance 1.567e+08
Ho: a_score(group==Group 1) = a_score(group==Group 2)
z = -1.076
Prob > |z| = 0.2818
Have I used the correct test?
Kind regards,
Gwinyai
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