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Module « scipy.stats »

Fonction dirichlet_multinomial - module scipy.stats

Signature de la fonction dirichlet_multinomial

def dirichlet_multinomial(alpha, n, seed=None) 

Description

help(scipy.stats.dirichlet_multinomial)

A Dirichlet multinomial random variable.

The Dirichlet multinomial distribution is a compound probability
distribution: it is the multinomial distribution with number of trials
`n` and class probabilities ``p`` randomly sampled from a Dirichlet
distribution with concentration parameters ``alpha``.

Methods
-------
logpmf(x, alpha, n):
    Log of the probability mass function.
pmf(x, alpha, n):
    Probability mass function.
mean(alpha, n):
    Mean of the Dirichlet multinomial distribution.
var(alpha, n):
    Variance of the Dirichlet multinomial distribution.
cov(alpha, n):
    The covariance of the Dirichlet multinomial distribution.

Parameters
----------
alpha : array_like
    The concentration parameters. The number of entries along the last axis
    determines the dimensionality of the distribution. Each entry must be
    strictly positive.
n : int or array_like
    The number of trials. Each element must be a strictly positive integer.
seed : {None, int, np.random.RandomState, np.random.Generator}, optional
    Used for drawing random variates.
    If `seed` is `None`, the `~np.random.RandomState` singleton is used.
    If `seed` is an int, a new ``RandomState`` instance is used, seeded
    with seed.
    If `seed` is already a ``RandomState`` or ``Generator`` instance,
    then that object is used.
    Default is `None`.

See Also
--------
scipy.stats.dirichlet : The dirichlet distribution.
scipy.stats.multinomial : The multinomial distribution.

References
----------
.. [1] Dirichlet-multinomial distribution, Wikipedia,
       https://www.wikipedia.org/wiki/Dirichlet-multinomial_distribution

Examples
--------
>>> from scipy.stats import dirichlet_multinomial

Get the PMF

>>> n = 6  # number of trials
>>> alpha = [3, 4, 5]  # concentration parameters
>>> x = [1, 2, 3]  # counts
>>> dirichlet_multinomial.pmf(x, alpha, n)
0.08484162895927604

If the sum of category counts does not equal the number of trials,
the probability mass is zero.

>>> dirichlet_multinomial.pmf(x, alpha, n=7)
0.0

Get the log of the PMF

>>> dirichlet_multinomial.logpmf(x, alpha, n)
-2.4669689491013327

Get the mean

>>> dirichlet_multinomial.mean(alpha, n)
array([1.5, 2. , 2.5])

Get the variance

>>> dirichlet_multinomial.var(alpha, n)
array([1.55769231, 1.84615385, 2.01923077])

Get the covariance

>>> dirichlet_multinomial.cov(alpha, n)
array([[ 1.55769231, -0.69230769, -0.86538462],
       [-0.69230769,  1.84615385, -1.15384615],
       [-0.86538462, -1.15384615,  2.01923077]])

Alternatively, the object may be called (as a function) to fix the
`alpha` and `n` parameters, returning a "frozen" Dirichlet multinomial
random variable.

>>> dm = dirichlet_multinomial(alpha, n)
>>> dm.pmf(x)
0.08484162895927579

All methods are fully vectorized. Each element of `x` and `alpha` is
a vector (along the last axis), each element of `n` is an
integer (scalar), and the result is computed element-wise.

>>> x = [[1, 2, 3], [4, 5, 6]]
>>> alpha = [[1, 2, 3], [4, 5, 6]]
>>> n = [6, 15]
>>> dirichlet_multinomial.pmf(x, alpha, n)
array([0.06493506, 0.02626937])

>>> dirichlet_multinomial.cov(alpha, n).shape  # both covariance matrices
(2, 3, 3)

Broadcasting according to standard NumPy conventions is supported. Here,
we have four sets of concentration parameters (each a two element vector)
for each of three numbers of trials (each a scalar).

>>> alpha = [[3, 4], [4, 5], [5, 6], [6, 7]]
>>> n = [[6], [7], [8]]
>>> dirichlet_multinomial.mean(alpha, n).shape
(3, 4, 2)

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