scipy.stats.chi2

scipy.stats.chi2 = <scipy.stats._continuous_distns.chi2_gen object at 0x7f6169d16310>

A chi-squared continuous random variable.

Continuous random variables are defined from a standard form and may require some shape parameters to complete its specification. Any optional keyword parameters can be passed to the methods of the RV object as given below:

Parameters:

x : array_like

quantiles

q : array_like

lower or upper tail probability

df : array_like

shape parameters

loc : array_like, optional

location parameter (default=0)

scale : array_like, optional

scale parameter (default=1)

size : int or tuple of ints, optional

shape of random variates (default computed from input arguments )

moments : str, optional

composed of letters [‘mvsk’] specifying which moments to compute where ‘m’ = mean, ‘v’ = variance, ‘s’ = (Fisher’s) skew and ‘k’ = (Fisher’s) kurtosis. Default is ‘mv’.

Alternatively, the object may be called (as a function) to fix the shape,

location, and scale parameters returning a “frozen” continuous RV object:

rv = chi2(df, loc=0, scale=1)

• Frozen RV object with the same methods but holding the given shape, location, and scale fixed.

Notes

The probability density function for chi2 is:

chi2.pdf(x, df) = 1 / (2*gamma(df/2)) * (x/2)**(df/2-1) * exp(-x/2)

Examples

>>> from scipy.stats import chi2
>>> import matplotlib.pyplot as plt
>>> fig, ax = plt.subplots(1, 1)

Calculate a few first moments:

>>> df = 55
>>> mean, var, skew, kurt = chi2.stats(df, moments='mvsk')

Display the probability density function (pdf):

>>> x = np.linspace(chi2.ppf(0.01, df),
...               chi2.ppf(0.99, df), 100)
>>> ax.plot(x, chi2.pdf(x, df),
...          'r-', lw=5, alpha=0.6, label='chi2 pdf')

Alternatively, freeze the distribution and display the frozen pdf:

>>> rv = chi2(df)
>>> ax.plot(x, rv.pdf(x), 'k-', lw=2, label='frozen pdf')

Check accuracy of cdf and ppf:

>>> vals = chi2.ppf([0.001, 0.5, 0.999], df)
>>> np.allclose([0.001, 0.5, 0.999], chi2.cdf(vals, df))
True

Generate random numbers:

>>> r = chi2.rvs(df, size=1000)

And compare the histogram:

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

(Source code)

Methods

rvs(df, loc=0, scale=1, size=1)	Random variates.
pdf(x, df, loc=0, scale=1)	Probability density function.
logpdf(x, df, loc=0, scale=1)	Log of the probability density function.
cdf(x, df, loc=0, scale=1)	Cumulative density function.
logcdf(x, df, loc=0, scale=1)	Log of the cumulative density function.
sf(x, df, loc=0, scale=1)	Survival function (1-cdf — sometimes more accurate).
logsf(x, df, loc=0, scale=1)	Log of the survival function.
ppf(q, df, loc=0, scale=1)	Percent point function (inverse of cdf — percentiles).
isf(q, df, loc=0, scale=1)	Inverse survival function (inverse of sf).
moment(n, df, loc=0, scale=1)	Non-central moment of order n
stats(df, loc=0, scale=1, moments='mv')	Mean(‘m’), variance(‘v’), skew(‘s’), and/or kurtosis(‘k’).
entropy(df, loc=0, scale=1)	(Differential) entropy of the RV.
fit(data, df, loc=0, scale=1)	Parameter estimates for generic data.
expect(func, df, 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(df, loc=0, scale=1)	Median of the distribution.
mean(df, loc=0, scale=1)	Mean of the distribution.
var(df, loc=0, scale=1)	Variance of the distribution.
std(df, loc=0, scale=1)	Standard deviation of the distribution.
interval(alpha, df, loc=0, scale=1)	Endpoints of the range that contains alpha percent of the distribution