scipy.stats.skewcauchy#

scipy.stats.skewcauchy = <scipy.stats._continuous_distns.skewcauchy_gen object>[source]#

A skewed Cauchy random variable.

As an instance of the rv_continuous class, skewcauchy 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

cauchy: Cauchy distribution

Notes

The probability density function for skewcauchy is:

\[f(x) = \frac{1}{\pi \left(\frac{x^2}{\left(a\, \text{sign}(x) + 1 \right)^2} + 1 \right)}\]

for a real number \(x\) and skewness parameter \(-1 < a < 1\).

When \(a=0\), the distribution reduces to the usual Cauchy distribution.

The probability density above is defined in the “standardized” form. To shift and/or scale the distribution use the loc and scale parameters. Specifically, skewcauchy.pdf(x, a, loc, scale) is identically equivalent to skewcauchy.pdf(y, a) / scale with y = (x - loc) / scale. Note that shifting the location of a distribution does not make it a “noncentral” distribution; noncentral generalizations of some distributions are available in separate classes.

References

1: “Skewed generalized t distribution”, Wikipedia https://en.wikipedia.org/wiki/Skewed_generalized_t_distribution#Skewed_Cauchy_distribution

Examples

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

Calculate the first four moments:

>>> a = 0.5
>>> mean, var, skew, kurt = skewcauchy.stats(a, moments='mvsk')

Display the probability density function (pdf):

>>> x = np.linspace(skewcauchy.ppf(0.01, a),
...                 skewcauchy.ppf(0.99, a), 100)
>>> ax.plot(x, skewcauchy.pdf(x, a),
...        'r-', lw=5, alpha=0.6, label='skewcauchy pdf')

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

Freeze the distribution and display the frozen pdf:

>>> rv = skewcauchy(a)
>>> ax.plot(x, rv.pdf(x), 'k-', lw=2, label='frozen pdf')

Check accuracy of cdf and ppf:

>>> vals = skewcauchy.ppf([0.001, 0.5, 0.999], a)
>>> np.allclose([0.001, 0.5, 0.999], skewcauchy.cdf(vals, a))
True

Generate random numbers:

>>> r = skewcauchy.rvs(a, size=1000)

And compare the histogram:

>>> ax.hist(r, density=True, histtype='stepfilled', alpha=0.2)
>>> ax.legend(loc='best', frameon=False)
>>> plt.show()

../../_images/scipy-stats-skewcauchy-1.png

Methods

rvs(a, loc=0, scale=1, size=1, random_state=None)	Random variates.
pdf(x, a, loc=0, scale=1)	Probability density function.
logpdf(x, a, loc=0, scale=1)	Log of the probability density function.
cdf(x, a, loc=0, scale=1)	Cumulative distribution function.
logcdf(x, a, loc=0, scale=1)	Log of the cumulative distribution function.
sf(x, a, loc=0, scale=1)	Survival function (also defined as `1 - cdf`, but sf is sometimes more accurate).
logsf(x, a, loc=0, scale=1)	Log of the survival function.
ppf(q, a, loc=0, scale=1)	Percent point function (inverse of `cdf` — percentiles).
isf(q, a, loc=0, scale=1)	Inverse survival function (inverse of `sf`).
moment(n, a, loc=0, scale=1)	Non-central moment of order n
stats(a, loc=0, scale=1, moments=’mv’)	Mean(‘m’), variance(‘v’), skew(‘s’), and/or kurtosis(‘k’).
entropy(a, loc=0, scale=1)	(Differential) entropy of the RV.
fit(data)	Parameter estimates for generic data. See scipy.stats.rv_continuous.fit for detailed documentation of the keyword arguments.
expect(func, args=(a,), loc=0, scale=1, lb=None, ub=None, conditional=False, kwds)**	Expected value of a function (of one argument) with respect to the distribution.
median(a, loc=0, scale=1)	Median of the distribution.
mean(a, loc=0, scale=1)	Mean of the distribution.
var(a, loc=0, scale=1)	Variance of the distribution.
std(a, loc=0, scale=1)	Standard deviation of the distribution.
interval(alpha, a, loc=0, scale=1)	Endpoints of the range that contains fraction alpha [0, 1] of the distribution

scipy.stats.semicircular

scipy.stats.skewnorm