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Nesting Probabilistic Programs
Proceedings of the 34th Conference on Uncertainty in Artificial Intelligence, PMLR R16:248-257, 2018.
Abstract
We formalize the notion of nesting probabilistic programming queries and investigate the result- ing statistical implications. We demonstrate that while query nesting allows the definition of models which could not otherwise be ex- pressed, such as those involving agents reason- ing about other agents, existing systems take approaches which lead to inconsistent estimates. We show how to correct this by delineating pos- sible ways one might want to nest queries and asserting the respective conditions required for convergence. We further introduce a new on- line nested Monte Carlo estimator that makes it substantially easier to ensure these conditions are met, thereby providing a simple framework for designing statistically correct inference en- gines. We prove the correctness of this online estimator and show that, when using the recom- mended setup, its asymptotic variance is always better than that of the equivalent fixed estimator, while its bias is always within a factor of two.