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Theory of Statistics - George Mason University

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496 6 Statistical Inference Based on Likelihood<br />

L(α;x)<br />

Exponential(α,1)<br />

α<br />

]<br />

(<br />

x(1)<br />

In some cases, it turns out that the estimation <strong>of</strong> a subset <strong>of</strong> the parameters<br />

does not depend on the value <strong>of</strong> some other subset. A good method <strong>of</strong><br />

estimation <strong>of</strong> β in a linear model X = Zβ + ɛ where the residuals ɛ have<br />

a common variance σ 2 and zero correlation can be performed equally well<br />

no matter what the value <strong>of</strong> σ 2 is. (The Gauss-Markov theorem tells us that<br />

the least-squares method yields a good estimator.) If the residuals are independently<br />

distributed as normals with a common variance, we can formulate<br />

the problem as a problem in maximum likelihood estimation. The MLE for β<br />

(which just happens to be the same as the LSE) can be thought <strong>of</strong> in terms <strong>of</strong><br />

a pr<strong>of</strong>ile likelihood, because a particular value <strong>of</strong> σ 2 could be chosen a priori.<br />

(This is <strong>of</strong> course not necessary because the maximum or the likelihood with<br />

respect to β occurs at the same point regardless <strong>of</strong> the value <strong>of</strong> σ 2 .)<br />

Conditional Likelihood<br />

When there is a nuisance parameter for which we have a sufficient statistic,<br />

a simple approach is to use the PDF conditional on the sufficient statistic to<br />

form the likelihood function for the parameter <strong>of</strong> interest. After doing this, the<br />

MLE procedure continues as in the usual case. If the PDFs can be factored<br />

so that one factor includes θ2 and some function <strong>of</strong> the sample, S(x), and the<br />

other factor, given S(x), is free <strong>of</strong> θ2, then this factorization can be carried<br />

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

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