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Are there any parametric assumptions for Linear Mixed Models - lmer (multivariate model)? I saw a few videos or journals that have reported results without resulting in any assumptions? any answer with reference will be appreciated

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The fact that they didn't mention assumptions doesn't mean there aren't any. Many strong assumptions are made, e.g.,

  • The right hand side of the model is structurally correct with regard to nonlinearities and interactions
  • The overall error term is normal with constant variance
  • The random effects are normal with constant variance
  • The within-subject correlation is constant. If analyzing longitudinal data this is unlikely to be true, as typically correlations are stronger for measurements taken closer together in time.
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