Cumulative Distribution Function (CDF)

There are many ways of specifying distributions. We have sometimes used a table to display the distribution of a random variable $X$. At other times we have written $P(X = k)$ as a formula for each possible value $k$ of $X$.

Another useful function that encapsulates all the information about the distribution of $X$ is called the cumulative distribution function of $X$. That's a real mouthful, so it is usually abbreviated to the cdf of $X$.

Let's see what the cdf is in an example.

Suppose $X$ has the distribution given below. It happens to be the binomial $(3, 1/2)$ distribution, but that is not important for this discussion.

The cdf of $X$ is another way of providing the same information. It is the function $F$ defined by

$$ F(x) ~ = ~ P(X \le x), ~~~~~ -\infty < x < \infty $$

The cdf of the random variable $X$, evaluated at the point $x$, is the chance that the value of $X$ is at most $x$.

The gold area in the probability histogram below is $F(2)$.

We will sometimes use the loose term "left hand probabilities" to denote values of $F$. Each of those values $F(x)$ is the area of the bar at $x$ as well as all the bars to the left. In the figure above,

$$ F(2) ~ = ~ P(X \le 2) ~ = ~ P(X = 0) + P(X = 1) + P(X = 2) $$

We will now augment the distribution table by a row containing some values of $F$. These are obtained obtained by adding the probabilities $P(X=x)$ successively from the left end. This cumulative sum is the reason for the name of the function.

Notice that we can recover $P(X=x)$ from values of $F$ as follows. For integer $x$,

$$ P(X = x) ~ = ~ P(X \le x) - P(X \le x-1) ~ = ~ F(x) - F(x-1) $$

These calculations show that if you know the distribution of $X$ then you know the cdf of $X$, and vice versa. The distribution and the cdf contain the same information.

Graph of the CDF

The graph of every cdf has some properties that are easy to see:

• The graph is non-decreasing.
• The values are on the vertical axis are probabilities and hence are between 0 and 1.
• The graph starts out at or near 0 for large negative values of $x$, and ends up at or near 1 for large positive values of $x$.

You can see all these properties in the graph of the function $F$ defined above.

You can see also that the graph has flat parts and jumps.

• Flat parts: These are in-between the possible values of $X$ and also beyond the possible values in both directions. Since $X$ is a non-negative variable, for all negative $x$ we have $F(x) = P(X \le x) = 0$. Since $X$ is always at most 3, for all $x > 3$ we have $F(x) = P(X \le x) = 1$. For $x$ in between two possible values, say $x = 1.6$, we have $F(1.6) = P(X \le 1.6) = P(X \le 1) = F(1)$. So for $x \in [1, 2)$, the graph is flat at $F(1)$. You can explain all the other flat parts analogously.

• Jumps: The graph has a jump (or discontinuity) at each possible value $x$ of $X$. That is, the graph jumps at each value $x$ such that $P(X = x) > 0$. The size of the jump at $x$ is equal to $P(X = x)$. For example, $P(X = 2)$ is the size of the jump at $x=2$, which is $0.875 - 0.5 = 0.375 = 3/8$.

Computation

We will have many uses for the cdf in this course. In fact, we have already used it several times without giving it a name.

For example, the chance of at most 3 sixes in 20 rolls of a die is given by the binomial formula and the addition rule:

$$ P(\text{at most 3 sixes in 20 rolls}) ~ = ~ \sum_{k=0}^3 \binom{20}{k}(1/6)^k(5/6)^{20-k} $$

This can be computed as

Let $X$ be the number of sixes in 20 rolls. Then $X$ has the binomial $(20, 1/6)$ distribution. The answer above is $P(X \le 3)$. That's $F(3)$ where $F$ is the cdf of $X$.

The stats module includes a cdf method that allows us to obtain the answer directly without summing.

The expression stats.binom.cdf(k, n, p) evaluates to $F(k)$ where $F$ is the cdf of a binomial $(n, p)$ random variable.

So our answer can also be found as follows.

Probabilities are frequently computed as sums. The cdf is a very useful tool for doing this, so stats provides a cdf method associated with each distribution.

You can use stats.hypergeom.cdf(k, N, G, n) to find the value of $F(k)$ for a random variable that has the hypergeometric $(N, G, n)$ distribution.

For example, recall that a standard deck contains 52 cards of which 12 are face cards. The chance of more than 5 face cards in a bridge hand of 13 cards dealt from a standard deck is

$$ \begin{align*} &P(\text{more than 5 face cards in a hand of 13 cards}) \\ &= ~ 1 - P(\text{at most 5 face cards in the hand}) \\ &= ~ 1 - \sum_{k=0}^{5} \frac{\binom{12}{k}\binom{40}{13-k}}{\binom{52}{13}} \end{align*} $$

Now you can get the numerical value by using stats.hypergeom.pmf: