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When the *r* parameter is an integer, the negative
binomial pdf is

where *q* = 1 – *p*.
When *r* is not an integer, the binomial coefficient
in the definition of the pdf is replaced by the equivalent expression

In its simplest form (when *r* is an integer),
the negative binomial distribution models the number of failures *x* before
a specified number of successes is reached in a series of independent,
identical trials. Its parameters are the probability of success in
a single trial, *p*, and the number of successes, *r*.
A special case of the negative binomial distribution, when *r* =
1, is the geometric
distribution, which models the number of failures before the
first success.

More generally, *r* can take on non-integer
values. This form of the negative binomial distribution has no interpretation
in terms of repeated trials, but, like the Poisson distribution, it is useful
in modeling count data. The negative binomial distribution is more
general than the Poisson distribution because it has a variance that
is greater than its mean, making it suitable for count data that do
not meet the assumptions of the Poisson distribution. In the limit,
as *r* increases to infinity, the negative binomial
distribution approaches the Poisson distribution.

Suppose you are collecting data on the number of auto accidents on a busy highway, and would like to be able to model the number of accidents per day. Because these are count data, and because there are a very large number of cars and a small probability of an accident for any specific car, you might think to use the Poisson distribution. However, the probability of having an accident is likely to vary from day to day as the weather and amount of traffic change, and so the assumptions needed for the Poisson distribution are not met. In particular, the variance of this type of count data sometimes exceeds the mean by a large amount. The data below exhibit this effect: most days have few or no accidents, and a few days have a large number.

accident = [2 3 4 2 3 1 12 8 14 31 23 1 10 7 0]; m = mean(accident) v = var(accident)

m = 8.0667 v = 79.3524

The negative binomial distribution is more general than the
Poisson, and is often suitable for count data when the Poisson is
not. The function `nbinfit` returns
the maximum likelihood estimates (MLEs) and confidence intervals for
the parameters of the negative binomial distribution. Here are the
results from fitting the `accident` data:

[phat,pci] = nbinfit(accident)

phat = 1.0060 0.1109 pci = 0.2152 0.0171 1.7968 0.2046

It is difficult to give a physical interpretation in this case to the individual parameters. However, the estimated parameters can be used in a model for the number of daily accidents. For example, a plot of the estimated cumulative probability function shows that while there is an estimated 10% chance of no accidents on a given day, there is also about a 10% chance that there will be 20 or more accidents.

plot(0:50,nbincdf(0:50,phat(1),phat(2)),'.-'); xlabel('Accidents per Day') ylabel('Cumulative Probability')

Compute and plot the pdf using four different values for the parameter `r`, the desired number of successes: `.1`, `1`, `3`, and `6`. In each case, the probability of success `p` is `.5`.

x = 0:10; plot(x,nbinpdf(x,.1,.5),'s-', ... x,nbinpdf(x,1,.5),'o-', ... x,nbinpdf(x,3,.5),'d-', ... x,nbinpdf(x,6,.5),'^-'); legend({'r = .1' 'r = 1' 'r = 3' 'r = 6'}) xlabel('x') ylabel('f(x|r,p)')

The plot shows that the negative binomial distribution can take on a variety of shapes, ranging from very skewed to nearly symmetric, depending on the value of `r`.

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