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STATISTICS 512 TECHNIQUES OF MATHEMATICS FOR ...

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34<br />

the mean squared error. Wehave<br />

Ã<br />

[z] =Q 0 Q<br />

0<br />

[y] = 1 Q 1 R 1 θ<br />

Q 0 2 Q 1R 1 θ<br />

!<br />

=<br />

Ã<br />

R1 θ<br />

0<br />

!<br />

<br />

and, using “cov[Ay] =Acov[y] A 0 ”(how?) we<br />

get<br />

cov [z] =Q 0 cov [y] Q = Q 0 2 IQ = 2 I;<br />

hence the elements +1 of z 2 have mean<br />

zero and [ ]= h <br />

2 i<br />

= 2 . Thus ˆ 2 is<br />

unbiased:<br />

hˆ 2i = <br />

⎡<br />

⎣P +1<br />

2 <br />

− <br />

⎤<br />

⎦ = 2 <br />

• Unrelated but nonetheless useful facts: inverses<br />

and determinants of matrices in block form. If P<br />

and Q are nonsingular, then<br />

det<br />

Ã<br />

P<br />

R<br />

S<br />

Q<br />

!<br />

= |P|·|Q − RP −1 S|<br />

= |Q|·|P − SQ −1 R|

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