# Tag Archives: Law of Total Probability

## Conditional Probability and Expectation

Conditional probability:

$\Pr(X\mid Y) = \displaystyle \frac{\Pr(X \cap Y)}{\Pr(Y)}$

Bayes Theorem:

$\Pr(A\mid B) = \displaystyle \frac{\Pr(B \mid A)\Pr(A)}{\Pr(B)}$

for continuous distributions:

$f_X(x\mid y) = \displaystyle \frac{f_Y(y \mid x)f_X(x)}{f_Y(y)}$

Recall for a joint distribution function $f(x,y)$,

$f_X(x) = \displaystyle \int_{-\infty}^\infty {f(x,y)dy}$

Law of Total Probability:  Suppose $\displaystyle \sum_{i=1}^n B_i = 1$ and $\Pr(B_i \cap B_j) = 0$ for $i \ne j$, then for any event $A$,

$\begin{array}{rl} \Pr(A) &= \displaystyle \sum_{i=1}^n \Pr(A \cap B_i) \\ &= \displaystyle \sum_{i=1}^n \Pr(B_i)\Pr(A\mid B_i) \end{array}$

In many cases, you will need to use the law of total probability in conjunction with Bayes Theorem to find $P(A)$ or $P(B)$.

For a continuous distribution:

$\Pr(A) = \displaystyle \int\Pr(A\mid x)f(x)dx$

Conditional Mean:

$E_X[X] = E_Y[E_X[X\mid Y]]$