Abstract
Let (Ω,F, P) be a probability space and A, B ∈F events such that P(B) > 0. In this section we seek to formalize the intuitive content of the phrase “probability of A given (or, equivalently, “conditioned on”) B”. The words “given B ” imply prior knowledge of the fact that the sample point ω lies in B. It is then natural to consider the reduced probability space (Ω, Fb, Pb) with ΩB = B,FB = A∩B A ∈ F (the “trace σ-field”), PB(C) = P(C)/P(B) for C ∈ TB, and define the probability of A given B, denoted by P(A/B), as PB(A ∩ B) = P(AB)/P(B). The reader may convince himself of the reasonableness of this procedure by evaluating, say, the probability of two consecutive heads in four tosses of a fair coin given the knowledge that the number of heads was even, using the principle of insufficient reason.
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© 1995 Springer Science+Business Media New York
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Borkar, V.S. (1995). Conditioning and Martingales. In: Probability Theory. Universitext. Springer, New York, NY. https://doi.org/10.1007/978-1-4612-0791-7_3
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DOI: https://doi.org/10.1007/978-1-4612-0791-7_3
Publisher Name: Springer, New York, NY
Print ISBN: 978-0-387-94558-3
Online ISBN: 978-1-4612-0791-7
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