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Theory of Statistics - George Mason University

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540 7 Statistical Hypotheses and Confidence Sets<br />

Randomized confidence Sets<br />

For discrete distributions, as we have seen, sometimes to achieve a test <strong>of</strong> a<br />

specified size, we had to use a randomized test.<br />

Confidence sets may have exactly the same problem – and solution – in<br />

forming confidence sets for parameters in discrete distributions. We form randomized<br />

confidence sets. The idea is the same as in randomized tests, and<br />

we will discuss randomized confidence sets in the context <strong>of</strong> hypothesis tests<br />

below.<br />

Pivotal Functions<br />

A straightforward way to form a confidence interval is to use a function <strong>of</strong> the<br />

sample that also involves the parameter <strong>of</strong> interest, but that does not involve<br />

any nuisance parameters. This kind <strong>of</strong> function is called a pivotal function.<br />

The confidence interval is then formed by separating the parameter from the<br />

sample values, as in Example 3.24 on page 294.<br />

Example 7.14 Confidence Interval for a Quantile<br />

***distribution free<br />

For a given parameter and family <strong>of</strong> distributions there may be multiple<br />

pivotal values. For purposes <strong>of</strong> statistical inference, such considerations as<br />

unbiasedness and minimum variance may guide the choice <strong>of</strong> a pivotal value<br />

to use.<br />

Approximate Pivot Values<br />

It may not be possible to identify a pivotal quantity for a particular parameter.<br />

In that case, we may seek an approximate pivot. A function is asymptotically<br />

pivotal if a sequence <strong>of</strong> linear transformations <strong>of</strong> the function is pivotal in the<br />

limit as n → ∞.<br />

*** nuisance parameters ***** find consistent estimator<br />

If the distribution <strong>of</strong> T is known, c1 and c2 can be determined. If the<br />

distribution <strong>of</strong> T is not known, some other approach must be used. A common<br />

method is to use some numerical approximation to the distribution. Another<br />

method is to use bootstrap samples from the ECDF.<br />

Relation to Acceptance Regions <strong>of</strong> Hypothesis Tests<br />

A test at the α level has a very close relationship with a 1 −α level confidence<br />

set.<br />

When we test the hypothesis H0 : θ ∈ ΘH0 at the α level, we form<br />

a critical region for a test statistic or rejection region for the values <strong>of</strong> the<br />

observable X. This region is such that the probability that the test statistic<br />

is in it is ≤ α.<br />

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

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