Testing the Information Matrix Equality with Robust Estimators



Clearly, σn 1, since Student’s t tends to N(0,1) as pn → ∞. Rewrite (16)
as

E Wʌ = bc,
ΓV √Γ+ε Л  c

where

ε = σn 1 + σnl, Л 2/Pn W + σn, R.

Expanding ρc ( Z/ √1 + ε) around ε = 0 gives

bc = E [ρc(Z)] 1E [(ZPc(Z)ε] + 1E [(Z2PC(Z) + 3C(Z))ε2]
2                 8

+o ( E ( ε2 )).

Since E [ρc(Z)] = bc, E(ε) = σ2n — 1 and E(ε2) = (σ2n — 1)2 + ‰σn + o(pn 1 ),
we obtain,

bc  =  bc - 2(σn - 1)e [Zρ'c(z)] + 4~~σne [z2ρ'c(z) + 3Zρc(z)]

2                      4pn

+o ( σn-1 ,pn1).

So

σn = 1 + 2⅛ + o ( P-1),                 (17)

2 pn

where

ς e [Z2ρc(Z) + 3Z^c(Z)]

1            E [Z(Z)]        .

Let ( m 1 ,m 2 ,m 3 ) ' = Wm ( Y ; θn ) with

/ u2 1 — d(u4 6u2 + 3) ʌ

W m ( Y ; θ ) = I          u3 3 u          I .

     u4 6 u2 + 3      /

By the symmetry of Student’s t distribution, EFn (ff 2) = 0. Furthermore,

EFn (Y2)


σn2


1


EFn( γ 2) σn + o ( P-1)


Pn

pn 2


2∑1 1 + o ( pn 1)
2
Pn

4 Σ1

2 pn


+ o ( P-1),


(18)


27




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