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I have a model based on a dataset that respects all linear model assumptions except for homoscedasticity. When I just ignore the problem of heteroscedasticity, the p-value, for the interaction with group, in my model is <.00001. I definitely know that there is something as per my previous studies and the literature in this field. However, I would like to be honest regarding my analyses and assumptions. Is this assumption really needed if the other 3 main ones are respected (independence, linearity, absence of collinearity) for the interpretation of the p-value in the mixed effects models?

When I run the following on my lmer model called mod:

plot(fitted(mod),residuals(mod))

enter image description here

I get a cone shape distribution. I then try to log transform it, and recheck the model, for the interaction with group in my model the p value goes to .40. Quite a jump! My data comes brain activity from patients and healthy individual, just to clarify.

This is my model:

lmer(value ~ dist*group + (1|patientnumber), dat1)

This is how I obtained the p-value:

Anova(mod)

Kindly advise.

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  • $\begingroup$ Not enough information here to advice regarding transformations or use of a GLMM. However, there is also the option of modeling variance in dependence of a variance covariate with package nlme. $\endgroup$
    – Roland
    Feb 17 '15 at 18:44
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(Posting as an answer so it gets carried over during migration rather than disappearing with comments.)

Inferences about interactions are extremely sensitive to log-transformation. It completely changes the meaning of the interaction, from additive to multiplicative. You will indeed get better advice regarding this on CrossValidated, although I would strongly recommend that you try to post your data, or at least an informative picture of your data, when asking a question there (or editing the version of this that gets migrated there).

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