# scipy.stats.poisson#

scipy.stats.poisson = <scipy.stats._discrete_distns.poisson_gen object>[source]#

A Poisson discrete random variable.

As an instance of the rv_discrete class, poisson object inherits from it a collection of generic methods (see below for the full list), and completes them with details specific for this particular distribution.

Notes

The probability mass function for poisson is:

$f(k) = \exp(-\mu) \frac{\mu^k}{k!}$

for $$k \ge 0$$.

poisson takes $$\mu \geq 0$$ as shape parameter. When $$\mu = 0$$, the pmf method returns 1.0 at quantile $$k = 0$$.

The probability mass function above is defined in the “standardized” form. To shift distribution use the loc parameter. Specifically, poisson.pmf(k, mu, loc) is identically equivalent to poisson.pmf(k - loc, mu).

Examples

>>> import numpy as np
>>> from scipy.stats import poisson
>>> import matplotlib.pyplot as plt
>>> fig, ax = plt.subplots(1, 1)


Calculate the first four moments:

>>> mu = 0.6
>>> mean, var, skew, kurt = poisson.stats(mu, moments='mvsk')


Display the probability mass function (pmf):

>>> x = np.arange(poisson.ppf(0.01, mu),
...               poisson.ppf(0.99, mu))
>>> ax.plot(x, poisson.pmf(x, mu), 'bo', ms=8, label='poisson pmf')
>>> ax.vlines(x, 0, poisson.pmf(x, mu), colors='b', lw=5, alpha=0.5)


Alternatively, the distribution object can be called (as a function) to fix the shape and location. This returns a “frozen” RV object holding the given parameters fixed.

Freeze the distribution and display the frozen pmf:

>>> rv = poisson(mu)
>>> ax.vlines(x, 0, rv.pmf(x), colors='k', linestyles='-', lw=1,
...         label='frozen pmf')
>>> ax.legend(loc='best', frameon=False)
>>> plt.show()


Check accuracy of cdf and ppf:

>>> prob = poisson.cdf(x, mu)
>>> np.allclose(x, poisson.ppf(prob, mu))
True


Generate random numbers:

>>> r = poisson.rvs(mu, size=1000)


Methods

 rvs(mu, loc=0, size=1, random_state=None) Random variates. pmf(k, mu, loc=0) Probability mass function. logpmf(k, mu, loc=0) Log of the probability mass function. cdf(k, mu, loc=0) Cumulative distribution function. logcdf(k, mu, loc=0) Log of the cumulative distribution function. sf(k, mu, loc=0) Survival function (also defined as 1 - cdf, but sf is sometimes more accurate). logsf(k, mu, loc=0) Log of the survival function. ppf(q, mu, loc=0) Percent point function (inverse of cdf — percentiles). isf(q, mu, loc=0) Inverse survival function (inverse of sf). stats(mu, loc=0, moments=’mv’) Mean(‘m’), variance(‘v’), skew(‘s’), and/or kurtosis(‘k’). entropy(mu, loc=0) (Differential) entropy of the RV. expect(func, args=(mu,), loc=0, lb=None, ub=None, conditional=False) Expected value of a function (of one argument) with respect to the distribution. median(mu, loc=0) Median of the distribution. mean(mu, loc=0) Mean of the distribution. var(mu, loc=0) Variance of the distribution. std(mu, loc=0) Standard deviation of the distribution. interval(confidence, mu, loc=0) Confidence interval with equal areas around the median.