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Asymptotic Methods in Statistical Inference - Statistics Centre

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35<br />

• Delta method. Suppose that is ( 2<br />

),<br />

i.e. √ <br />

( − ) → ∼ (0 2 ), and that<br />

0 () existsandis6= 0. Def<strong>in</strong>e by<br />

( ) − () =( − ) 0 ()+ <br />

We claim that = ( − ) (ifthe are<br />

constants then this is the MVT), so √ =<br />

( √ ( − )) = ( (1)) = (1) (assigned).<br />

This gives<br />

√ ( ( ) − ()) = √ ( − ) 0 ()+ (1)<br />

<br />

→ (0 h 0 () i 2<br />

)<br />

by Slutsky.<br />

Proofofclaim:<br />

<br />

− = ( )−()<br />

−<br />

− 0 () =<br />

( ), say. Def<strong>in</strong>e () =0,sothat is cont<strong>in</strong>uous<br />

at . Now → (why?), so<br />

<br />

( ) → () =0.<br />

By Slutsky’s theorem, we also have<br />

√ ( ( ) − ()) <br />

0 → (0 1)<br />

(ˆ )<br />

<br />

as long as → , ˆ → , and 0 is cont<strong>in</strong>uous<br />

at .

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