条件期望与重期望
条件期望的定义:
(E(x|y)=int_{-infty}^{infty}xf(x|y)dx)(连续)
(E(x|y)=sumlimits_ix_i ho(X=x_i|Y=y_i))(离散)
重期望的性质
(1.E(E(g(x)|Y))=int_{-infty}^{infty}E(E(g(x)|Y))f_{Y}(y)dy)
=(int_{-infty}^{infty}[int_{-infty}^{infty}g(x)f(x|y)dx]f_{Y}(y)dy)
=(int_{-infty}^{infty}int_{-infty}^{infty}g(x)f(x|y)f_{Y}(y)dxdy)
=(int_{-infty}^{infty}int_{-infty}^{infty}g(x)f(x|y)dxdy)
=(E(g(x)))
(2.E(h(y)g(x)|Y))