# numpy.random.weibull

random.weibull(a, size=None)

  Draw samples from a Weibull distribution.

  Draw samples from a 1-parameter Weibull distribution with the given shape parameter a.

  \[X = (-ln(U))^{1/a}\]

  Here, U is drawn from the uniform distribution over (0,1].

  The more common 2-parameter Weibull, including a scale parameter \(\lambda\) is just \(X = \lambda(-ln(U))^{1/a}\).

  Note

  New code should use the `weibull` method of a `default_rng()` instance instead; please see the Quick Start.

  Parameters
    a float or array_like of floats

      Shape parameter of the distribution. Must be nonnegative.

    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 Weibull distribution.

  See also

  `scipy.stats.weibull_max`
  `scipy.stats.weibull_min`
  `scipy.stats.genextreme`
  `gumbel`
  `Generator.weibull`

    which should be used for new code.

  Notes

  The Weibull (or Type III asymptotic extreme value distribution for smallest values, SEV Type III, or Rosin-Rammler distribution) is one of a class of Generalized Extreme Value (GEV) distributions used in modeling extreme value problems. This class includes the Gumbel and Frechet distributions.

  The probability density for the Weibull distribution is

  \[p(x) = \frac{a} {\lambda}(\frac{x}{\lambda})^{a-1}e^{-(x/\lambda)^a},\]

  where \(a\) is the shape and \(\lambda\) the scale.

  The function has its peak (the mode) at \(\lambda(\frac{a-1}{a})^{1/a}\).

  When `a = 1`, the Weibull distribution reduces to the exponential distribution.

  References

  1

    Waloddi Weibull, Royal Technical University, Stockholm, 1939 “A Statistical Theory Of The Strength Of Materials”, Ingeniorsvetenskapsakademiens Handlingar Nr 151, 1939, Generalstabens Litografiska Anstalts Forlag, Stockholm.

  2

    Waloddi Weibull, “A Statistical Distribution Function of Wide Applicability”, Journal Of Applied Mechanics ASME Paper 1951.

  3

    Wikipedia, “Weibull distribution”, https://en.wikipedia.org/wiki/Weibull_distribution

  Examples

  Draw samples from the distribution:

      >>> a = 5. # shape
      >>> s = np.random.weibull(a, 1000)

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

      >>> import matplotlib.pyplot as plt
      >>> x = np.arange(1,100.)/50.
      >>> def weib(x,n,a):
      ...     return (a / n) * (x / n)**(a - 1) * np.exp(-(x / n)**a)

      >>> count, bins, ignored = plt.hist(np.random.weibull(5.,1000))
      >>> x = np.arange(1,100.)/50.
      >>> scale = count.max()/weib(x, 1., 5.).max()
      >>> plt.plot(x, weib(x, 1., 5.)*scale)
      >>> plt.show()

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