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I would like to compare group means among 4 groups and analyse changes in their characteristics over time. I have a panel data set which is unbalanced. My sample sizes are not equal and therefore violate the assumptions of a mixed model anova? Any suggestions on how I can proceed?

Also I wanted confirmation if a mixed model anova is the same as a multilevel modelling.

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For an unbalanced fixed effect ANOVA designs a Type II or Type III sum of squares approach is used. The picture's more complicated with unbalanced mixed model ANOVA, I don't know if there's a remedy - this previous post might help:

Unbalanced mixed effect ANOVA for repeated measures

Might be best to run a generalized linear model (GLM) regression with mixed effects rather than an ANOVA.

A mixed model ANOVA isn't necessarily the same as multilevel modeling. The mixed model is a crossed effect model, where each factor contains the different levels of the other factors. In multilevel (or nested or hierarchical) modeling, certain levels of a factor are embedded within one factor but not others (like different subjects within each clinic).

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