Shapiro-Wilk different results

ravello · Post by **ravello** » Mon Nov 21, 2022 1:31 am

Hello
Responding to a subscriber of my channel about jamovi, I realized that when I calculate Shapiro-Wilk in 2 ways a) descriptive / normality and b) T-test / normality (moretest) I get different values, attached image.
greetings and thanks in advance for your answer

decaux · Post by **decaux** » Mon Nov 21, 2022 7:16 am

I think, in the first analysis you are looking at all the dependent variable data together, while in the second analysis, the S-W statistic takes into account the independent variable

ravello · Post by **ravello** » Mon Nov 21, 2022 3:20 pm

All the analysis is on the same variable, it will be then that the calculation of S-W changes when it is on an independent t-test.
This is the database used:
https://github.com/ravellom/DataScience ... t-test.omv

MAgojam · Post by **MAgojam** » Tue Nov 22, 2022 1:04 pm

Hey @ravello,

The shapiro-wilk test is performed on the residues and not on the groups themselves, because putting all the residues together and testing is accepted practice.
This means that if the residuals of one group are not normally distributed, the residuals of all groups combined will also not be normally distributed.
Would you like to take a look here, at an old post, where Jth and I had given an example and given an answer.
https://forum.jamovi.org/viewtopic.php?f=6&t=1350&hilit=Shapiro#p4741

Ceers,
Maurizio

jamovi