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Independent cross-entropy production is conditionally unbiased
Statement
Use Independent-pilot cross-entropy proposal fitting and production. Let and let be the event that the pilot succeeds and the algorithm's exact admissibility checker accepts the selected parameter . On , the checker guarantees and integrability under of the piecewise contribution defined in the algorithm. Define to equal the returned production estimate on and zero off . Then
In particular, the production estimate is conditionally unbiased almost surely on the successful, admissible pilot event. This gives no probability-of-success, proposal-optimization, finite-variance, logarithmic-efficiency, or total-work guarantee.
Facts & Assumptions
Given: The pilot sigma-algebra , the successful and admissible event , the frozen parameter , the independent production sample, and the support and integrability conditions for certified by the algorithm's checker.
Independent-pilot cross-entropy proposal fitting and production: After a successful pilot the selected is frozen, and the production sample is IID from , independent of the pilot conditional on that parameter.
Ordinary importance sampling is unbiased: For a fixed proposal with support on the target event and an integrable weighted contribution, the ordinary importance-sampling mean equals the target expectation.
Proof
The pilot outcome space need not be finite. For each , the score is -measurable, and is finite. Therefore the success event, the exact-checker acceptance event, and the lexicographically least maximizing selector on success are -measurable. Partition into the finitely many events . Conditional independence of the production sample from the pilot given , as specified in [F1], means that on each its conditional law given is IID .
On each , the checker gives the support and integrability hypotheses for [F2]. That theorem gives conditional expectation for each production contribution given , and linearity gives the same conditional expectation for their average. Since the finitely many partition and off , this proves almost surely.
Depends on
Used by
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Sources
- Patrick Laub, Algorithms for Rare Event Estimation, Short Course, Lecture 2, Cross-Entropy Method (standard reference, not scraped)
- Gino Biondini, An Introduction to Rare Event Simulation and Importance Sampling, Handbook of Statistics 33, Chapter 2, `5 (standard reference, not scraped)