Should MLE or REML used to estimate the model? To test the null and the grouped average model, MLE should be used for likelihood comparison. Need to check whether the variance component is used and whether the sample size is large enough.
Given a large sample, the bias of the variance estimation is small for ML. In addition, for model comparison ML is more suitable as exact same fixed effect is required for REML.
Should MLE or REML used to estimate the model? To test the null and the grouped average model, MLE should be used for likelihood comparison. Need to check whether the variance component is used and whether the sample size is large enough.