SciPy

scipy.stats.bernoulli

scipy.stats.bernoulli(*args, **kwds) = <scipy.stats._discrete_distns.bernoulli_gen object>[source]

A Bernoulli discrete random variable.

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

\[\begin{split}f(k) = \begin{cases}1-p &\text{if } k = 0\\ p &\text{if } k = 1\end{cases}\end{split}\]

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

bernoulli takes \(p\) as shape parameter, 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, bernoulli.pmf(k, p, loc) is identically equivalent to bernoulli.pmf(k - loc, p).

Examples

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

Calculate a few first moments:

>>> p = 0.3
>>> mean, var, skew, kurt = bernoulli.stats(p, moments='mvsk')

Display the probability mass function (pmf):

>>> x = np.arange(bernoulli.ppf(0.01, p),
...               bernoulli.ppf(0.99, p))
>>> ax.plot(x, bernoulli.pmf(x, p), 'bo', ms=8, label='bernoulli pmf')
>>> ax.vlines(x, 0, bernoulli.pmf(x, 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 = bernoulli(p)
>>> ax.vlines(x, 0, rv.pmf(x), colors='k', linestyles='-', lw=1,
...         label='frozen pmf')
>>> ax.legend(loc='best', frameon=False)
>>> plt.show()
../_images/scipy-stats-bernoulli-1_00_00.png

Check accuracy of cdf and ppf:

>>> prob = bernoulli.cdf(x, p)
>>> np.allclose(x, bernoulli.ppf(prob, p))
True

Generate random numbers:

>>> r = bernoulli.rvs(p, size=1000)

Methods

rvs(p, loc=0, size=1, random_state=None)

Random variates.

pmf(k, p, loc=0)

Probability mass function.

logpmf(k, p, loc=0)

Log of the probability mass function.

cdf(k, p, loc=0)

Cumulative distribution function.

logcdf(k, p, loc=0)

Log of the cumulative distribution function.

sf(k, p, loc=0)

Survival function (also defined as 1 - cdf, but sf is sometimes more accurate).

logsf(k, p, loc=0)

Log of the survival function.

ppf(q, p, loc=0)

Percent point function (inverse of cdf — percentiles).

isf(q, p, loc=0)

Inverse survival function (inverse of sf).

stats(p, loc=0, moments=’mv’)

Mean(‘m’), variance(‘v’), skew(‘s’), and/or kurtosis(‘k’).

entropy(p, loc=0)

(Differential) entropy of the RV.

expect(func, args=(p,), loc=0, lb=None, ub=None, conditional=False)

Expected value of a function (of one argument) with respect to the distribution.

median(p, loc=0)

Median of the distribution.

mean(p, loc=0)

Mean of the distribution.

var(p, loc=0)

Variance of the distribution.

std(p, loc=0)

Standard deviation of the distribution.

interval(alpha, p, loc=0)

Endpoints of the range that contains alpha percent of the distribution

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