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Testing for normality for each groups in the ANOVA

Dear colleagues,

I am exploring JASP for first time and I am not sure how to test for assumption of normality for each groups when you conduct an ANOVA. There is a Q-Q plot but for all the data, not stratified by groups, there is not a Saphiro-wilk test as in the t-test. Is there any method to proceed?

Thanks!

Comments

  • Hi mrvallejo,

    I'll ask Johnny (who has done most of the frequentist ANOVA work). It seems to me that what the ANOVA model assumes is normality of the residuals, but Johnny will know best.

    Cheers,

    E.J.

  • edited March 25

    Hi mrvallejo,

    The normality assumption of the ANOVA concerns the distribution of the residuals, not the raw data itself. Because the residuals are the differences between the observed dependent variable and the predicted dependent variable, we can show the qq-plot for all groups combined, which is commonly done.

    I find this exchange particularly enlightening: https://stats.stackexchange.com/questions/6350/anova-assumption-normality-normal-distribution-of-residuals

    Basically, if you plot the residuals instead of the raw data, you can fit all residuals to the same normal distribution, whereas if you were to do this for the raw data, you would need to fit it to different normal distributions per group.

    Kind regards,

    Johnny

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