Partial effects in mixed models

Dear all,

I would like to test the effect of an explanatory variable after removing the effect of another one. I thought about calculating the model with the first explanatory 
variable only, then take the model residuals and use the residuals as response variable to test the effect of the second explanatory variable. However, I do not 
know if this is possible for a model containing random effects. Maybe it doesn't make sense anyway, but if it is possible, should I include the random effects in the 
second model (residuals as response variable) or not, since variance explained by random effects should also have been accounted for in the first model?

Thank you for your help

Val?rie
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