12.5 Mean, Variance, and Sampling Distribution of the Least Squares Estimators 
and 
We noted above (via the Gauss–Markov Theorem) that 
and 
(hence the sampling distributions of 
and 
are centered right on 
and 
, respectively). In addition, it can be demonstrated that
where 
is the unknown variance of the random error term ε. Since 
is unknown and must be estimated from the sample data, let ...
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