e' e / σ2 = u' M' M u / σ2
             
= u' M u / σ2 ~ χ r2
r = rank( M )
  = trace( M )
  = trace ( I - P )
  = trace ( I ) - trace ( P )
r = n - k
e' e / σ2 = u' M u / σ2 ~ χ n - k2
根據理論,E( χ2 ) = Degree of Freedom ,那麼
E( e' e / σ2 ) = n - k
E[ e' e / ( n - k ) ] = σ2