The reason for using R (Restricted method) is because the alternative M (Maximum method) can have bias about covariance (level-2 variance in our application) and when the number of group unit is relatively small, so this is a real threat.
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The reason for using R (Restricted method) is because the alternative M (Maximum method) can have bias about covariance (level-2 variance in our application) and when the number of group unit is relatively small, so this is a real threat.