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.

See also

hypergeom, nbinom, nhypergeom

Notes

The probability mass function for binom is:

\[f(k) = \binom{n}{k} p^k (1-p)^{n-k}\]

for \(k \in \{0, 1, \dots, n\}\), \(0 \leq p \leq 1\)

binom takes \(n\) and \(p\) as shape parameters, where \(p\) is the probability of a single success and \(1-p\) is the probability of a single failure.

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 the first four 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(k, n, p, loc=0)	Probability mass function.
logpmf(k, n, p, loc=0)	Log of the probability mass function.
cdf(k, n, p, loc=0)	Cumulative distribution function.
logcdf(k, n, p, loc=0)	Log of the cumulative distribution function.
sf(k, n, p, loc=0)	Survival function (also defined as 1 - cdf, but sf is sometimes more accurate).
logsf(k, 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 fraction alpha [0, 1] of the distribution

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