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Theory of Statistics - George Mason University

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3.4 Invariant and Equivariant Statistical Procedures 285<br />

Location-Scale Equivariant Estimation<br />

Location-scale equivariance involves the combination <strong>of</strong> the two separate developments.<br />

The basic transformations are location and scale: X = bX + c<br />

and ˜ θ = bθ + c.<br />

The loss function (3.128) for estimation <strong>of</strong> the scale parameter is invariant<br />

to both location and scale transformations, and the estimator <strong>of</strong> the scale<br />

must have the form <strong>of</strong> (3.130).<br />

In order for the loss function for estimation <strong>of</strong> the location parameter to<br />

be invariant under a location and scale transformation, the loss function must<br />

be <strong>of</strong> the form<br />

L(µ, a) = L((a − µ)/σ), (3.137)<br />

and the location estimator must have the property<br />

T(bx + c) = b r T(x) + c. (3.138)<br />

Analysis <strong>of</strong> these estimators does not involve anything fundamentally different<br />

from combinations <strong>of</strong> the ideas discussed separately for the location<br />

and scale cases.<br />

Equivariant Estimation in a Normal Family<br />

MRE estimation has particular relevance to the family <strong>of</strong> normal distributions,<br />

which is a location-scale group family.<br />

Example 3.23 Equivariant Estimation in a Normal Family<br />

Suppose X1, X2, . . ., Xn are iid as N(µ, σ 2 ) distribution, and consider the<br />

problem <strong>of</strong> estimation <strong>of</strong> µ and σ 2 .<br />

************************<br />

1<br />

Tσ2(X) =<br />

n + 1<br />

**** compare MLE, minimum MSE<br />

n<br />

(Xi − X) 2<br />

i=1<br />

(3.139)<br />

Tµ(X) = X (3.140)<br />

The MRE estimator <strong>of</strong> the location under a convex and even loss function<br />

<strong>of</strong> the form (3.138) and MRE estimator <strong>of</strong> the scale under a loss <strong>of</strong> the<br />

form (3.130) are independent <strong>of</strong> each other. Another interesting fact is that<br />

in location families that have densities with respect to Lebesgue measure and<br />

with finite variance, the risk <strong>of</strong> a MRE location estimator with scaled squarederror<br />

loss is larger in the normal family than in any other location-scale group<br />

family.<br />

<strong>Theory</strong> <strong>of</strong> <strong>Statistics</strong> c○2000–2013 James E. Gentle

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