As shown in the image below, how does A turn into B? What is the intuitive logic behind the disappearance of sigma y? Thanks!!
summation sign of y in a conditional probability proof
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$\begingroup$
probability
probability-theory
summation
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0You've "summed out" all dependence on $y$, essentially. Try looking at a specific example. – 2012-12-02
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0This is exactly correct. Formally this is called the Law of Total Probability. – 2012-12-02