Test of Normality (Shapiro-Wilk)

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by Geograman » Sat Jun 20, 2020 2:30 pm

Hello, I have a question about the Test of Normality (Shapiro-Wilk) in an Independent Samples T-Test. Normally you would check both groups for normal distribution as a requirement for the T-test. So you would get 2 results of the Shapiro-Wilk test. One result for each group. But I have seen that you can also do a test for normal distribution directly in the T-test. But this test seems to give only one result for both groups and not for each group separately. My question now is what exactly this result indicates.
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by MAgojam » Sat Jun 20, 2020 9:35 pm

Hi, @Geograman.

Some time ago, Jantonie posted here:
a problem with using the Shapiro-Wilk normality test.
Suggesting, when having two samples, to perform the normality test not on the scores, but rather on the differences within the group.

Here as a reminder for the jamovi team:

I think Jonathon subsequently applied Jantonie's suggestion.

I am attaching a Screenshot, with a small script in the Rj module that applies this tip.
ScreenShot_S-W.PNG (95.61 KiB) Viewed 229 times

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by jonathon » Sat Jun 20, 2020 11:29 pm

hi geograman,

the shapiro-wilk test is performed on the residuals, and not on the groups themselves. i do occasionally bump into people who want to test the groups separately. if you want to do that, you can do that in descriptives.

but putting all the residuals together and testing is accepted practice.

(it follows that if the residuals from one group is not normally distributed, then the residuals from all the groups combined will not be normally distributed either).


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by Geograman » Sun Jun 21, 2020 9:27 am

Hi, thanks a lot, now I understand what jamovi is doing here. :)
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