0

I have worked out $f_X(x),f_Y(y),f_{X|Y}(x|y) \ \text{and} \ f_{Y|X}(y|x)$

I wish to verify that $E(E(X|Y))=E(X)$

For $E(X)$ I will be calculating:

$$\int_{I_x}xf_X(x) \ dx$$

and for $E(E(X|Y))$ I am not so sure, but currently I think:

$$E(X|Y)=\int_{I_x}xf_{X|Y}(x|y) \ dx $$

but I am not sure what to do from here. Any help would be good.

($I_x$ is the domain on which $x$ is defined for the continuous random variable)

1 Answers1

1

(Beware: case sensitivity.)

$\mathrm I_X$ appears to be the support for the random variable $X$.   Likewise you would have $\mathrm I_Y$ as the support for the random variable $Y$.

Similarly you should use $\mathrm I_{X\mid Y=y}$ to be the conditional support for the variable given a particular value, $y$, for the random variable $Y$.

And thus:

$$\begin{align}\mathsf E(\mathsf E(X\mid Y)) ~&=~ \int_{\mathrm I_Y}\;f_Y(y)\color{navy}{\int_{\mathrm I_{X\mid Y=y}} x\,f_{X\mid Y}(x\mid y)\operatorname d x}\operatorname d y \\ &=~ \iint_{\mathrm I_{X\times Y}} x\,f_{X,Y}(x,y)\operatorname d (x,y) & \text{Fubini/Tonelli} \\ &=~ \text{something}\end{align}$$

And then you are done, okay.

Graham Kemp
  • 129,094