Module « numpy.random »

Fonction pareto - module numpy.random

Signature de la fonction pareto

Description

pareto.__doc__

        pareto(a, size=None)

        Draw samples from a Pareto II or Lomax distribution with
        specified shape.

        The Lomax or Pareto II distribution is a shifted Pareto
        distribution. The classical Pareto distribution can be
        obtained from the Lomax distribution by adding 1 and
        multiplying by the scale parameter ``m`` (see Notes).  The
        smallest value of the Lomax distribution is zero while for the
        classical Pareto distribution it is ``mu``, where the standard
        Pareto distribution has location ``mu = 1``.  Lomax can also
        be considered as a simplified version of the Generalized
        Pareto distribution (available in SciPy), with the scale set
        to one and the location set to zero.

        The Pareto distribution must be greater than zero, and is
        unbounded above.  It is also known as the "80-20 rule".  In
        this distribution, 80 percent of the weights are in the lowest
        20 percent of the range, while the other 20 percent fill the
        remaining 80 percent of the range.

        .. note::
            New code should use the ``pareto`` method of a ``default_rng()``
            instance instead; please see the :ref:`random-quick-start`.

        Parameters
        ----------
        a : float or array_like of floats
            Shape of the distribution. Must be positive.
        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.  If size is ``None`` (default),
            a single value is returned if ``a`` is a scalar.  Otherwise,
            ``np.array(a).size`` samples are drawn.

        Returns
        -------
        out : ndarray or scalar
            Drawn samples from the parameterized Pareto distribution.

        See Also
        --------
        scipy.stats.lomax : probability density function, distribution or
            cumulative density function, etc.
        scipy.stats.genpareto : probability density function, distribution or
            cumulative density function, etc.
        Generator.pareto: which should be used for new code.

        Notes
        -----
        The probability density for the Pareto distribution is

        .. math:: p(x) = \frac{am^a}{x^{a+1}}

        where :math:`a` is the shape and :math:`m` the scale.

        The Pareto distribution, named after the Italian economist
        Vilfredo Pareto, is a power law probability distribution
        useful in many real world problems.  Outside the field of
        economics it is generally referred to as the Bradford
        distribution. Pareto developed the distribution to describe
        the distribution of wealth in an economy.  It has also found
        use in insurance, web page access statistics, oil field sizes,
        and many other problems, including the download frequency for
        projects in Sourceforge [1]_.  It is one of the so-called
        "fat-tailed" distributions.

        References
        ----------
        .. [1] Francis Hunt and Paul Johnson, On the Pareto Distribution of
               Sourceforge projects.
        .. [2] Pareto, V. (1896). Course of Political Economy. Lausanne.
        .. [3] Reiss, R.D., Thomas, M.(2001), Statistical Analysis of Extreme
               Values, Birkhauser Verlag, Basel, pp 23-30.
        .. [4] Wikipedia, "Pareto distribution",
               https://en.wikipedia.org/wiki/Pareto_distribution

        Examples
        --------
        Draw samples from the distribution:

        >>> a, m = 3., 2.  # shape and mode
        >>> s = (np.random.pareto(a, 1000) + 1) * m

        Display the histogram of the samples, along with the probability
        density function:

        >>> import matplotlib.pyplot as plt
        >>> count, bins, _ = plt.hist(s, 100, density=True)
        >>> fit = a*m**a / bins**(a+1)
        >>> plt.plot(bins, max(count)*fit/max(fit), linewidth=2, color='r')
        >>> plt.show()