# Simulating random variables from a discrete distribution

I have the following discrete distribution where $p$ is a known constant:

$p(x,p)= \frac{(1-p)^3}{p(1+p)}x^2p^x , (0<p<1), x=0, 1, 2, \ldots$ .

How can I sample from this distribution?

– Tim
May 23 '16 at 15:03
• Does this even sum to 1 for some value of $p$? (I doubt so) May 23 '16 at 15:19
• @Elvis It does sum to 1. May 23 '16 at 15:41
• @MatthewGunn My bad. May 24 '16 at 6:43

This answer develops a simple procedure to generate values from this distribution. It illustrates the procedure, analyzes its scope of application (that is, for which $p$ it might be considered a practical method), and provides executable code.

### The Idea

Because

$$x^2 = 2\binom{x}{2} + \binom{x}{1},$$

consider the distributions $f_{p;m}$ given by

$$f_{p;m}(x) \propto \binom{x}{m-1}p^x$$

for $m=3$ and $m=2$.

A recent thread on inverse sampling demonstrates that these distributions count the number of observations of independent Bernoulli$(1-p)$ variables needed before first seeing $m$ successes, with $x+1$ equal to that number. It also shows that the normalizing constant is

$$C(p;m)=\sum_{x=m-1}^\infty \binom{x}{m-1}p^x = \frac{p^{m-1}}{(1-p)^m}.$$

Consider the probabilities in the question,

$$x^2 p^x = \left( 2\binom{x}{2} + \binom{x}{1} \right)p^x = 2 \binom{x}{2}p^x + \binom{x}{1} p^x =2 C(p;3) f_{p;3}(x) + C(p;2) f_{p;2}(x).$$

Consequently, the given distribution is a mixture of $f_{p;3}$ and $f_{p;2}$. The proportions are as $$2C(p;3):C(p;2) = 2p:(1-p).$$ It is simple to sample from a mixture: generate an independent uniform variate $u$ and draw $x$ from $f_{p;2}$ when $u \lt (1-p)/(2p+1-p)$; that is, when $u(1+p) \lt 1-p$, and otherwise draw $x$ from $f_{p;3}$.

(It is evident that this method generalizes: many probability distributions where the chance of $x$ is of the form $P(x)p^x$ for a polynomial $P$, such as $P(x)=x^2$ here, can be expressed as a mixture of these inverse-sampling distributions.)

### The Algorithm

These considerations lead to the following simple algorithm to generate one realization of the desired distribution:

Let U ~ Uniform(0,1+p)
If (U < 1-p) then m = 2 else m = 3
x = 0
While (m > 0) {
x = x + 1
Let Z ~ Bernoulli(1-p)
m = m - Z
}
Return x-1 These histograms show simulations (based on 100,000 iterations) and the true distribution for a range of values of $p$.

### Analysis

How efficient is this? The expectation of $x+1$ under the distribution $f_{p;m}$ is readily computed; it equals $m/(1-p)$. Therefore the expected number of trials (that is, values of Z to generate in the algorithm) is

$$\left((1-p) \frac{2}{1-p} + (2p) \frac{3}{1-p}\right) / (1-p+2p) = 2 \frac{1+2p}{1-p^2}.$$

Add one more for generating U. The total is close to $3$ for small values of $p$. As $p$ approaches $1$, this count asymptotically is

$$1 + 2\frac{1 + 2p}{(1-p)(1+p)} \approx \frac{3}{1-p}.$$

This shows us that the algorithm will, on the average, be reasonably quick for $p \lt 2/3$ (taking up to ten easy steps) and not too bad for $p \lt 0.97$ (taking under a hundred steps).

### Code

Here is the R code used to implement the algorithm and produce the figures. A $\chi^2$ test will show that the simulated results do not differ significantly from the expected frequencies.

sample <- function(p) {
m <- ifelse(runif(1, max=1+p) < 1-p, 2, 3)
x <- 0
while (m > 0) {
x <- x + 1
m <- m - (runif(1) > p)
}
return(x-1)
}
n <- 1e5
set.seed(17)
par(mfcol=c(2,3))
for (p in c(1/5, 1/2, 9/10)) {

# Simulate and summarize.
x <- replicate(n, sample(p))
y <- table(x)

# Compute the true distribution for comparison.
k <- as.numeric(names(y))
theta <- sapply(k, function(i) i^2 * p^i) * (1-p)^3 / (p^2 + p)
names(theta) <- names(y)

# Plot both.
barplot(y/n, main=paste("Simulation for", format(p, digits=2)),
border="#00000010")
barplot(theta, main=paste("Distribution for", format(p, digits=2)),
border="#00000010")
}


@dsaxton's approach is known as inverse transform sampling and is probably the way to go for a problem like this. To be a bit more explicit, the approach is:

1. Draw $u$ from uniform distribution on (0,1).
2. Compute $x = F^{-1}(u)$ where $F^{-1}$ is the inverse of the cumulative distribution function.

Computing $x = F^{-1}(u)$ is equivalent to finding the integer $x$ that is the solution to: $$\text{minimize} \quad x \quad \text{subject to} \sum_{j=0}^x \frac{(1 - p)^3}{p(1+p)} j^2p^j \geq u$$

Quick pseudo code to do this numerically:

1. Construct a vector $\boldsymbol{m}$ such that $m_j = \frac{(1 - p)^3}{p(1+p)} j^2p^j$.
2. Create a vector $\boldsymbol{c}$ such that $c_j = \sum_{k=0}^j m_j$.
3. Find the minimum index $x$ such that $c_x \geq u$.

Draw $u$ from a uniform$(0, 1)$ distribution and let $x$ be the smallest value of $k$ for which $\sum_{j=0}^{k} \frac{(1 - p)^3}{p (1 + p)} j^2 p^j > u$. Then $x$ will be a realization from the desired distribution.

• May be clearer to write $x = \min k \text{ subject to } \sum_j^k \ldots > u$. As written, it's not terribly clear what is being minimized and that $x$ equals the optimal $k$. May 23 '16 at 16:10