Relating Binomial and Negative Binomial

The negative binomial distribution has a natural intepretation as a waiting time until the arrival of the rth success (when the parameter r is a positive integer). The waiting time refers to the number of independent Bernoulli trials needed to reach the rth success. This interpretation of the negative binomial distribution gives us a good way of relating it to the binomial distribution. For example, if the rth success takes place after k failed Bernoulli trials (for a total of k+r trials), then there can be at most r-1 successes in the first k+r trials. This tells us that the survival function of the negative binomial distribution is the cumulative distribution function (cdf) of a binomial distribution. In this post, we gives the details behind this observation. A previous post on the negative binomial distribution is found here.

A random experiment resulting in two distinct outcomes (success or failure) is called a Bernoulli trial (e.g. head or tail in a coin toss, whether or not the birthday of a customer is the first of January, whether an insurance claim is above or below a given threshold etc). Suppose a series of independent Bernoulli trials are performed until reaching the rth success where the probability of success in each trial is p. Let X_r be the number of failures before the occurrence of the rth success. The following is the probablity mass function of X_r.

\displaystyle (1) \ \ \ \ P(X_r=k)=\binom{k+r-1}{k} p^r (1-p)^k \ \ \ \ \ \ k=0,1,2,3,\cdots

Be definition, the survival function and cdf of X_r are:

\displaystyle (2) \ \ \ \ P(X_r > k)=\sum \limits_{j=k+1}^\infty \binom{j+r-1}{j} p^r (1-p)^j \ \ \ \ \ \ k=0,1,2,3,\cdots

\displaystyle (3) \ \ \ \ P(X_r \le k)=\sum \limits_{j=0}^k \binom{j+r-1}{j} p^r (1-p)^j \ \ \ \ \ \ k=0,1,2,3,\cdots

For each positive integer k, let Y_{r+k} be the number of successes in performing a sequence of r+k independent Bernoulli trials where p is the probability of success. In other words, Y_{r+k} has a binomial distribution with parameters r+k and p.

If the random experiment requires more than k failures to reach the rth success, there are at most r-1 successes in the first k+r trails. Thus the survival function of X_r is the same as the cdf of a binomial distribution. Equivalently, the cdf of X_r is the same as the survival function of a binomial distribution. We have the following:

\displaystyle \begin{aligned}(4) \ \ \ \ P(X_r > k)&=P(Y_{k+r} \le r-1) \\&=\sum \limits_{j=0}^{r-1} \binom{k+r}{j} p^j (1-p)^{k+r-j} \ \ \ \ \ \ k=0,1,2,3,\cdots \end{aligned}

\displaystyle \begin{aligned}(5) \ \ \ \ P(X_r \le k)&=P(Y_{k+r} > r-1) \ \ \ \ \ \ k=0,1,2,3,\cdots \end{aligned}

Remark
The relation (4) is analogous to the relationship between the Gamma distribution and the Poisson distribution. Recall that a Gamma distribution with shape parameter \alpha and scale parameter n, where n is a positive integer, can be interpreted as the waiting time until the nth change in a Poisson process. Thus, if the nth change takes place after time t, there can be at most n-1 arrivals in the time interval [0,t]. Thus the survival function of this Gamma distribution is the same as the cdf of a Poisson distribution. The relation (4) is analogous to the following relation.

\displaystyle (5) \ \ \ \ \int_t^\infty \frac{\alpha^n}{(n-1)!} \ x^{n-1} \ e^{-\alpha x} \ dx=\sum \limits_{j=0}^{n-1} \frac{e^{-\alpha t} \ (\alpha t)^j}{j!}

A previous post on the negative binomial distribution is found here.

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2 thoughts on “Relating Binomial and Negative Binomial

  1. Greetings! Very useful advice within this article! It’s
    the little changes which will make the most significant changes.
    Thanks for sharing!

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