RandomState.chisquare(df, size=None)

Draw samples from a chi-square distribution.

When df independent random variables, each with standard normal distributions (mean 0, variance 1), are squared and summed, the resulting distribution is chi-square (see Notes). This distribution is often used in hypothesis testing.


df : int

Number of degrees of freedom.

size : int or tuple of ints, optional

Output shape. If the given shape is, e.g., (m, n, k), then m * n * k samples are drawn. Default is None, in which case a single value is returned.


output : ndarray

Samples drawn from the distribution, packed in a size-shaped array.


ValueError :

When df <= 0 or when an inappropriate size (e.g. size=-1) is given.


The variable obtained by summing the squares of df independent, standard normally distributed random variables:

Q = \sum_{i=0}^{\mathtt{df}} X^2_i

is chi-square distributed, denoted

Q \sim \chi^2_k.

The probability density function of the chi-squared distribution is

p(x) = \frac{(1/2)^{k/2}}{\Gamma(k/2)}
x^{k/2 - 1} e^{-x/2},

where \Gamma is the gamma function,

\Gamma(x) = \int_0^{-\infty} t^{x - 1} e^{-t} dt.


[R142]NIST “Engineering Statistics Handbook”


>>> np.random.chisquare(2,4)
array([ 1.89920014,  9.00867716,  3.13710533,  5.62318272])