5.8.5.2. scipy.stats.binom¶

scipy.stats.binom = <scipy.stats._discrete_distns.binom_gen object>¶

A binomial discrete random variable.

As an instance of the rv_discrete class, binom 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 binom is:

binom.pmf(k) = choose(n, k) * p**k * (1-p)**(n-k)

for k in {0, 1,..., n}.

binom takes n and p as shape parameters.

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

Examples

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

Calculate a few first moments:

>>> n, p = 5, 0.4
>>> mean, var, skew, kurt = binom.stats(n, p, moments='mvsk')

Display the probability mass function (pmf):

>>> x = np.arange(binom.ppf(0.01, n, p),
...               binom.ppf(0.99, n, p))
>>> ax.plot(x, binom.pmf(x, n, p), 'bo', ms=8, label='binom pmf')
>>> ax.vlines(x, 0, binom.pmf(x, n, p), 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 = binom(n, p)
>>> 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 = binom.cdf(x, n, p)
>>> np.allclose(x, binom.ppf(prob, n, p))
True

Generate random numbers:

>>> r = binom.rvs(n, p, size=1000)

Methods

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