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Chapman–Robbins bound : ウィキペディア英語版
Chapman–Robbins bound
In statistics, the Chapman–Robbins bound or Hammersley–Chapman–Robbins bound is a lower bound on the variance of estimators of a deterministic parameter. It is a generalization of the Cramér–Rao bound; compared to the Cramér–Rao bound, it is both tighter and applicable to a wider range of problems. However, it is usually more difficult to compute.
The bound was independently discovered by John Hammersley in 1950, and by Douglas Chapman and Herbert Robbins in 1951.
== Statement ==
Let be an unknown, deterministic parameter, and let be a random variable, interpreted as a measurement of ''θ''. Suppose the probability density function of ''X'' is given by ''p''(''x''; ''θ''). It is assumed that ''p''(''x''; ''θ'') is well-defined and that for all values of ''x'' and ''θ''.
Suppose ''δ''(''X'') is an unbiased estimate of an arbitrary scalar function of ''θ'', i.e.,
:E_\ = g(\theta)\text\theta.\,
The Chapman–Robbins bound then states that
:\mathrm_(\delta(X)) \ge \sup_\Delta \frac - 1 \right )^2}.
Note that the denominator in the lower bound above is exactly the \chi^2-divergence of p(\cdot; \theta+\Delta) with respect to p(\cdot; \theta).

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