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Score (statistics) : ウィキペディア英語版
Score (statistics)
In statistics, the score, score function, efficient score〔Cox & Hinkley (1974), p 107〕 or informant indicates how sensitively a likelihood function L(\theta; X) depends on its parameter \theta. Explicitly, the score for \theta is the gradient of the log-likelihood with respect to \theta.
The score plays an important role in several aspects of inference. For example:
:
*in formulating a test statistic for a locally most powerful test;〔Cox & Hinkley (1974), p 113〕
:
*in approximating the error in a maximum likelihood estimate;〔Cox & Hinkley (1974), p 295〕
:
*in demonstrating the asymptotic sufficiency of a maximum likelihood estimate;〔
:
*in the formulation of confidence intervals;〔Cox & Hinkley (1974), p 222–3〕
:
*in demonstrations of the Cramér–Rao inequality.〔Cox & Hinkley (1974), p 254〕
The score function also plays an important role in computational statistics, as it can play a part in the computation of
maximum likelihood estimates.
==Definition==

The score or efficient score 〔 is the gradient (the vector of partial derivatives), with respect to some parameter \theta, of the logarithm (commonly the natural logarithm) of the likelihood function (the log-likelihood).
If the observation is X and its likelihood is L(\theta;X), then the score V can be found through the chain rule:
:
V \equiv V(\theta, X)
=
\frac \log L(\theta;X)
=
\frac \frac.

Thus the score V indicates the sensitivity of L(\theta;X) (its derivative normalized by its value). Note that V is a function of \theta and the observation X, so that, in general, it is not a statistic. However in certain applications, such as the score test, the score is evaluated at a specific value of \theta (such as a null-hypothesis value, or at the maximum likelihood estimate of \theta), in which case the result is a statistic.
In older literature, the term "linear score" may be used to refer to the score with respect to infinitesimal translation of a given density. This convention arises from a time when the primary parameter of interest was the mean or median of a distribution. In this case, the likelihood of an observation is given by a density of the form L(\theta;X)=f(X+\theta). The "linear score" is then defined as
:
V_
=
\frac \log f(X)


抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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