COMP4804: Algorithms II
Conditional Probability and Dependence

Here are my original hand-written notes on this topic.

Conditional Probability

The following definition tells us the conditional probability of an event $A$ given some event $B$: \begin{equation} \Pr\{A\mid B\} = \frac{\Pr\{A\cap B\}}{\Pr\{B\}} \label{conditional-i} \end{equation} Conditional probability answers questions like: "If I tell you that $B$ happened, then what is the probability that $A$ also happened?"

Example: Rolling a six-sided die

Roll 1 six-sided die, $D$:

Alternative Formulation of Conditional Probability

An equivalent formulation of \eqref{conditional-i} is obtained by multiplying both sides by $\Pr\{B\}$: \begin{equation} \Pr\{A\cap B\} = \Pr\{B\}\times\Pr\{A\mid B\} \label{conditional-ii} \end{equation} Equation \eqref{conditional-ii} is very useful because it gives us a formula for computing $\Pr\{A\cap B\}$ and it's also valid when $\Pr\{B\}=0$.


Two events $A$ and $B$ are independent if and only if \begin{equation} \Pr\{A\mid B\} = \Pr\{A\} \enspace . \label{independence-i} \end{equation} If $A$ and $B$ are not independent, then they are dependent. Using \eqref{conditional-ii}, we get the following equivalent definition of independence: $A$ and $B$ are indpendent if and only if \begin{equation} \Pr\{A\cap B\} = \Pr\{A\}\times\Pr\{B\} \enspace . \label{independence-ii} \end{equation} Note that this means that (in)dependence is symmetric: $A$ and $B$ are independent if and only if $B$ and $A$ are independent.

Example: Tossing two coins

Toss a gold coin and a silver coin: $\Pr\{TT\} = \Pr\{TH\} = \Pr\{HT\} = \Pr\{HH\} = 1/4$.

Are the events $A=\text{"the gold coin is heads"}$ and $B=\text{"the silver coin is heads"}$ independent? We have $A=\{HT,HH\}$, $B=\{TH,HH\}$, and $A\cap B=\{HH\}$ so \[ \Pr\{A\} = 2/4=1/2 \enspace , \] and \[ \Pr\{\text{gold coin is heads}\mid \text{silver coin is heads}\} = \frac{1/4}{2/4} = 1/2 \enspace . \] So $\Pr\{A\} = \Pr\{A\mid B\}$ and these two events are independent.

Example: Throwing two dice

Roll two dice, $D_1$ and $D_2$.

Let $A$ denote the event $D_1+D_2=7$ and let $B$ denote the event $D_1=4$.

From Lecture 1, \[ \Pr\{A\} = 6/36 = 1/6 \enspace . \] By definition \[ \Pr\{B\} = 1/6 \enspace . \] And \[ \Pr\{A\cap B\} = \Pr\{\text{$D_1=4$ and $D_2=3$}\} = 1/36 \enspace . \] So \[ $\Pr\{A\mid B\} = \frac{\Pr\{A\cap B\}}{\Pr\{B\}} = \frac{1/36}{1/6} = 1/6 = \Pr\{A\} \] So these two events are independent.

Rolling a 4 with the first die does not affect the probability that the sum of the two dice is 7.

Another kind of symmetry

If $A$ and $B$ are independent, then so are $A$ and $\overline{B}=U\setminus B$: \begin{align*} \Pr\{A\mid \overline{B}\} & = \frac{\Pr\{A\cap\overline{B}\}}{\Pr\{\overline{B}\}} \\ & = \frac{\Pr\{A\}-\Pr\{A \cap B\}}{1-\Pr\{B\}} \\ & = \frac{\Pr\{A\}-\Pr\{A\}\times\Pr\{B\}}{1-\Pr\{B\}} \text{ (using \eqref{independence-ii})}\\ & = \frac{\Pr\{A\}(1-\Pr\{B\})}{1-\Pr\{B\}} \\ & = \Pr\{A\} \end{align*}

So, in essence, we don't learn anything about $A$ when learn whether or not $B$ happened.

Independent Sets of events

We know what it means for two events $A$ and $B$ to be independent, but we also need a definition for more than just two events.

We say that a set of events $\{A_1,\ldots,A_n\}$ are independent if, for every $\{B_1,\ldots,B_r\}\subseteq\{A_1,\ldots,A_n\}$, \[ \Pr\{B_1\cap B_2\cap\cdots\cap B_r\} = \Pr\{B_1\}\times\Pr\{B_2\}\times\cdots\times\Pr\{B_r\} \enspace . \] This condition is time-consuming to check, so hopefully it's obvious in whatever application you're working on.

Example: Sequential circuits

A series circuit fails when any one of its components fail. If each of the $k$ components fails independently with probability $p$, then what is the probability that the circuit works? \begin{align*} \Pr\{\text{circuit works}\} &= \Pr\{\text{$C_1$ doesn't fail and … and $C_k$ doesn't fail}\} \\ &= (1-\Pr\{\text{$C_1$ fails}\}) \times \cdots\times (1-\Pr\{\text{$C_k$ fails}\}) \\ &= (1-p)^k \end{align*}

Example: Parallel circuits

A parallel circuit fails when all of its components fail. If each of the $k$ components fails independently with probability $p$, then what is the probability that the circuit works? \begin{align*} \Pr\{\text{circuit fails}\} &= \Pr\{\text{$C_1$ fails and … and $C_k$ fails}\} \\ &= p^k \end{align*} So the probability that the circuit works is $1-p^k$.

Markov's Inequality

Markov's Inequality: Let $X$ be a random variable that only takes on non-negative values. Then, for any $t>1$, \[ \Pr\{X \ge t\E[X]\} \le \frac{1}{t} \]

The proof of Markov's Inequality is basically: "If it were not true, then $\E[X]$ would be bigger than $\E[X]$". The following proof formalizes this:

Proof: Let \[ Y = \begin{cases}0 & \text{if $X\le t\E[X]$}\\ t\E[X] & \text{otherwise} \end{cases} \] Notice that $Y\le X$ so $\E[Y] \le \E[X]$. But we can compute $\E[Y]$ easily, giving \[ \E[X] \ge \E[Y] = t\E[X]\times\Pr\{X\ge t\E[X]\} \enspace , \] and dividing both sides by $t\E[X]$ gives, \[ 1/t \ge \Pr\{X\ge t\E[X]\} \enspace , \] as required. ∎

An Example application

Suppose we have a randomized algorithm $\mathcal{A}$ whose expected running-time on inputs of size $n$ is at most $f(n)$. Since running-times are non-negative, Markov's Inequality tells us that \[ \Pr\{\text{$\mathcal{B}$ runs for longer than $t f(n)$}\} \le 1/t \enspace . \] But we can do better! Create a new algorithm $\mathcal{B}$ defined as follows:

We assume that $\mathcal{A}$ makes new random choices each time it is restarted so that, for example, the probability that the first run of $\mathcal{A}$ runs longer than $2f(n)$ is independent of the probability that the second run of $\mathcal{A}$ runs for longer than $2f(n)$. Then, by Markov's Inequality and the independence of the runs of $\mathcal{A}$, we have for any even integer $t$, \[ \Pr\{\text{$\mathcal{B}$ runs for longer than $t f(n)$}\} \le (1/2)^{t/2} \enspace . \]

Imagine a nuclear reactor control system that will cause a meltdown if the underlying algorithm runs longer than $500f(n)$. Then the best we can say about $\mathcal{A}$ is \[ \Pr\{\text{meltdown using $\mathcal{A}$}\} \le \frac{1}{500} \enspace . \] But for $\mathcal{B}$ we have \[ \Pr\{\text{meltdown using $\mathcal{B}$}\} \le 2^{-250} = \frac{1}{180925139433306555349329664076074856020734351040063381311652475012364265062} \] Which would you rather rely on?