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A Remark on the Assumptions of Bayes' Theorem

By Janne V. Kujala

Abstract

We formulate simple equivalent conditions for the validity of Bayes' formula for conditional densities. We show that for any random variables X and Y (with values in arbitrary measurable spaces), the following are equivalent: 1. X and Y have a joint density w.r.t. a product measure \mu x \nu, 2. P_{X,Y} << P_X x P_Y, (here P_{.} denotes the distribution of {.}) 3. X has a conditional density p(x | y) w.r.t. a sigma-finite measure \mu, 4. X has a conditional distribution P_{X|Y} such that P_{X|y} << P_X for all y, 5. X has a conditional distribution P_{X|Y} and a marginal density p(x) w.r.t. a measure \mu such that P_{X|y} << \mu for all y. Furthermore, given random variables X and Y with a conditional density p(y | x) w.r.t. \nu and a marginal density p(x) w.r.t. \mu, we show that Bayes' formula p(x | y) = p(y | x)p(x) / \int p(y | x)p(x)d\mu(x) yields a conditional density p(x | y) w.r.t. \mu if and only if X and Y satisfy the above conditions. Counterexamples illustrating the nontriviality of the results are given, and implications for sequential adaptive estimation are considered.Comment: 10 page

Topics: Mathematics - Statistics Theory, 60A05, 60A10
Year: 2011
OAI identifier: oai:arXiv.org:1103.6136
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