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# Lec2 - Stat 150 Stochastic Processes Spring 2009 Lecture 2...

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Stat 150 Stochastic Processes Spring 2009 Lecture 2: Conditional Expectation Lecturer: Jim Pitman Some useful facts (assume all random variables here have ﬁnite mean square): E ( Y g ( X ) | X ) = g ( X ) E ( Y | X ) Y - E ( Y | X ) is orthogonal to E ( Y | X ), and orthogonal also to g ( X ) for every measurable function g . Since E ( Y | X ) is a measurable function of X , this characterizes E ( Y | X ) as the orthogonal projection of Y onto the linear space of all square-integrable random variables of the form g ( X ) for some measurable function g . Put another way g ( X ) = E ( Y | X ) minimizes the mean square prediction error E [( Y - g ( X )) 2 ] over all measurable functions g . ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± g ( X ) E ( Y | X ) ² Y Residual These facts can all be checked by computations as follows: Check orthogonality : E [( Y - E ( Y | X )) g ( X )] = E ( g ( X ) Y - g ( X ) E ( Y | X )) = E ( g ( X ) Y ) - E ( g ( X ) E ( Y | X )) = E ( E ( g ( X ) Y | X )) - E ( g ( X ) E ( Y | X )) = E ( g ( X ) E ( Y | X )) - E ( g ( X ) E ( Y | X )) = 0 Recall : V ar ( Y ) = E ( Y - E ( Y )) 2 and V ar ( Y | X ) = E ([ Y - E ( Y | X )] 2 | X ).

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Lec2 - Stat 150 Stochastic Processes Spring 2009 Lecture 2...

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