# Lesson 44 Covariance of Continuous Random Variables

## Theory

This lesson summarizes results about the covariance of continuous random variables. The statements of these results are exactly the same as for discrete random variables, but keep in mind that the expected values are now computed using integrals and p.d.f.s, rather than sums and p.m.f.s.

Definition 44.1 (Covariance) Let $$X$$ and $$Y$$ be random variables. Then, the covariance of $$X$$ and $$Y$$, symbolized $$\text{Cov}[X, Y]$$ is defined as $\begin{equation} \text{Cov}[X, Y] \overset{\text{def}}{=} E[(X - E[X])(Y - E[Y])]. \end{equation}$
Theorem 44.1 (Shortcut Formula for Covariance) The covariance can also be computed as: $\begin{equation} \text{Cov}[X, Y] = E[XY] - E[X]E[Y]. \tag{44.1} \end{equation}$

Example 44.1 (Covariance Between the First and Second Arrival Times) In Example 41.1, we saw that the joint distribution of the first arrival time $$X$$ and the second arrival time $$Y$$ in a Poisson process of rate $$\lambda = 0.8$$ is $f(x, y) = \begin{cases} 0.64 e^{-0.8 y} & 0 < x < y \\ 0 & \text{otherwise} \end{cases}.$ What is the covariance between $$X$$ and $$Y$$? Intuitively, we expect the covariance to be positive. The longer it takes for the first arrival to happen, the longer we will have to wait for the second arrival, since the second arrival has to happen after the first arrival. Let’s calculate the exact value of the covariance using the shortcut formula (44.1).

First, we need to calculate $$E[XY]$$. We do this using 2D LOTUS (43.1). If $$S = \{ (x, y): 0 < x < y \}$$ denotes the support of the distribution, then \begin{align*} E[XY] &= \iint_S xy \cdot 0.64 e^{-0.8 y}\,dx\,dy \\ &= \int_0^\infty \int_0^y xy \cdot 0.64 e^{-0.8 y}\,dx\,dy \\ &= \frac{75}{16}. \end{align*}

What about $$E[X]$$? We know that the first arrival follows an $$\text{Exponential}(\lambda=0.8)$$ distribution, so its expected value is $$1/\lambda = 1/0.8$$ seconds.

What about $$E[Y]$$? We showed in Example 43.3 that the $$r$$th arrival is expected to happen at $$r / 0.8$$ seconds (by linearity of expectation, since the $$r$$th arrival is the sum of the $$r$$ $$\text{Exponential}(\lambda=0.8)$$ interarrival times). Therefore, the secon arrival is expected to happen at $$E[Y] = 2 / 0.8$$ seconds.

Putting everything together, we have $\text{Cov}[X, Y] = E[XY] - E[X]E[Y] = \frac{75}{16} - \frac{1}{0.8} \cdot \frac{2}{0.8} = 1.5625.$

Theorem 44.2 (Properties of Covariance) Let $$X, Y, Z$$ be random variables, and let $$c$$ be a constant. Then:

1. Covariance-Variance Relationship: $$\displaystyle\text{Var}[X] = \text{Cov}[X, X]$$ (This was also Theorem 29.1.)
2. Pulling Out Constants:

$$\displaystyle\text{Cov}[cX, Y] = c \cdot \text{Cov}[X, Y]$$

$$\displaystyle\text{Cov}[X, cY] = c \cdot \text{Cov}[X, Y]$$

3. Distributive Property:

$$\displaystyle\text{Cov}[X + Y, Z] = \text{Cov}[X, Z] + \text{Cov}[Y, Z]$$

$$\displaystyle\text{Cov}[X, Y + Z] = \text{Cov}[X, Y] + \text{Cov}[X, Z]$$

4. Symmetry: $$\displaystyle\text{Cov}[X, Y] = \text{Cov}[Y, X]$$
5. Constants cannot covary: $$\displaystyle\text{Cov}[X, c] = 0$$.

Example 44.2 Here is an easier way to do Example 44.1, using properties of covariance.

Write $$Y = X + Z$$, where $$Z$$ is the time between the first and second arrivals. We know that $$Z$$ is $$\text{Exponential}(\lambda=0.8)$$ and independent of $$X$$, so $$\text{Cov}[X, Z] = 0$$.

Now, we have \begin{align*} \text{Cov}[X, Y] &= \text{Cov}[X, X + Z] = \underbrace{\text{Cov}[X, X]}_{\text{Var}[X]} + \underbrace{\text{Cov}[X, Z]}_0 = \frac{1}{0.8^2} = 1.5625, \end{align*} where in the last step we used the fact that the variance of an $$\text{Exponential}(\lambda)$$ random variable is $$1/\lambda^2$$. This matches the answer we got in Example 44.1, but it required much less calculation.

Example 44.3 (Standard Deviation of Arrival Times) In Example 43.3, we saw that the expected value of the $$r$$th arrival time in a Poisson process of rate $$\lambda=0.8$$ is $$r / 0.8$$. What is the standard deviation of the $$r$$th arrival time?

The $$r$$th arrival time $$S_r$$ is the sum of $$r$$ independent $$\text{Exponential}(\lambda=0.8)$$ random variables: $S_r = T_1 + T_2 + \ldots + T_r.$ By properties of covariance: \begin{align*} \text{Var}[S_r] &= \text{Cov}[S_r, S_r] \\ &= \text{Cov}[T_1 + T_2 + \ldots + T_r, T_1 + T_2 + \ldots + T_r] \\ &= \sum_{i=1}^r \underbrace{\text{Cov}[T_i, T_i]}_{\text{Var}[T_i]} + \sum_{i\neq j} \underbrace{\text{Cov}[T_i, T_j]}_0 \\ &= r \text{Var}[T_1] \\ &= r \frac{1}{0.8^2}. \end{align*}

Therefore, the standard deviation is $\text{SD}[S_r] = \frac{\sqrt{r}}{0.8}.$

## Essential Practice

1. Let $$A$$ be a $$\text{Exponential}(\lambda=1.5)$$ random variable, and let $$\Theta$$ be a $$\text{Uniform}(a=-\pi, b=\pi)$$ random variable. What is $$\text{Cov}[A\cos(\Theta + 2\pi s), A\cos(\Theta + 2\pi t)]$$?

2. In a standby system, a component is used until it wears out and is then immediately replaced by another, not necessarily identical, component. (The second component is said to be “in standby mode,” i.e., waiting to be used.) The overall lifetime of a standby system is just the sum of the lifetimes of its individual components. Let $$X$$ and $$Y$$ denote the lifetimes of the two components of a standby system, and suppose $$X$$ and $$Y$$ are independent exponentially distributed random variables with expected lifetimes 3 weeks and 4 weeks, respectively. Let $$T = X + Y$$, the lifetime of the standby system. What is the standard deviation of the lifetime of the system?

3. Let $$U_1, U_2, ..., U_n$$ be independent and identically distributed (i.i.d.) $$\text{Uniform}(a=0, b=1)$$ random variables. Let $$S_n = U_1 + ... + U_n$$ denote their sum. Calculate $$E[S_n]$$ and $$\text{SD}[S_n]$$ in terms of $$n$$.