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# scipy.stats.normal_inverse_gamma

scipy.stats.normal_inverse_gamma = <scipy.stats._multivariate.normal_inverse_gamma_gen object>

  Normal-inverse-gamma distribution.

  The normal-inverse-gamma distribution is the conjugate prior of a normal distribution with unknown mean and variance.

  Parameters:
    mu, lmbda, a, b array_like

      Shape parameters of the distribution. See notes.

    seed{None, int, np.random.RandomState, np.random.Generator}, optional

      Used for drawing random variates. If seed is None, the 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

  `norm`
  `invgamma`

  Notes

  The probability density function of `normal_inverse_gamma` is:

  \[f(x, \sigma^2; \mu, \lambda, \alpha, \beta) = \frac{\sqrt{\lambda}}{\sqrt{2 \pi \sigma^2}} \frac{\beta^\alpha}{\Gamma(\alpha)} \left( \frac{1}{\sigma^2} \right)^{\alpha + 1} \exp \left(- \frac{2 \beta + \lambda (x - \mu)^2} {2 \sigma^2} \right)\]

  where all parameters are real and finite, and \(\sigma^2 > 0\), \(\lambda > 0\), \(\alpha > 0\), and \(\beta > 0\).

  Methods `normal_inverse_gamma.pdf` and `normal_inverse_gamma.logpdf` accept x and s2 for arguments \(x\) and \(\sigma^2\). All methods accept mu, lmbda, a, and b for shape parameters \(\mu\), \(\lambda\), \(\alpha\), and \(\beta\), respectively.

  Added in version 1.15.

  References

  [1]

  Normal-inverse-gamma distribution, Wikipedia, https://en.wikipedia.org/wiki/Normal-inverse-gamma_distribution

  Examples

  Suppose we wish to investigate the relationship between the normal-inverse-gamma distribution and the inverse gamma distribution.

      >>> import numpy as np
      >>> from scipy import stats
      >>> import matplotlib.pyplot as plt
      >>> rng = np.random.default_rng()
      >>> mu, lmbda, a, b = 0, 1, 20, 20
      >>> norm_inv_gamma = stats.normal_inverse_gamma(mu, lmbda, a, b)
      >>> inv_gamma = stats.invgamma(a, scale=b)

  One approach is to compare the distribution of the s2 elements of random variates against the PDF of an inverse gamma distribution.

      >>> _, s2 = norm_inv_gamma.rvs(size=10000, random_state=rng)
      >>> bins = np.linspace(s2.min(), s2.max(), 50)
      >>> plt.hist(s2, bins=bins, density=True, label='Frequency density')
      >>> s2 = np.linspace(s2.min(), s2.max(), 300)
      >>> plt.plot(s2, inv_gamma.pdf(s2), label='PDF')
      >>> plt.xlabel(r'$\sigma^2$')
      >>> plt.ylabel('Frequency density / PMF')
      >>> plt.show()

  Similarly, we can compare the marginal distribution of s2 against an inverse gamma distribution.

      >>> from scipy.integrate import quad_vec
      >>> from scipy import integrate
      >>> s2 = np.linspace(0.5, 3, 6)
      >>> res = quad_vec(lambda x: norm_inv_gamma.pdf(x, s2), -np.inf, np.inf)[0]
      >>> np.allclose(res, inv_gamma.pdf(s2))
      True

  The sample mean is comparable to the mean of the distribution.

      >>> x, s2 = norm_inv_gamma.rvs(size=10000, random_state=rng)
      >>> x.mean(), s2.mean()
      (np.float64(-0.005254750127304425), np.float64(1.050438111436508))
      >>> norm_inv_gamma.mean()
      (np.float64(0.0), np.float64(1.0526315789473684))

  Similarly, for the variance:

      >>> x.var(ddof=1), s2.var(ddof=1)
      (np.float64(1.0546150578185023), np.float64(0.061829865266330754))
      >>> norm_inv_gamma.var()
      (np.float64(1.0526315789473684), np.float64(0.061557402277623886))

  Methods

  pdf(x, s2, mu=0, lmbda=1, a=1, b=1) | Probability density function.
  logpdf(x, s2, mu=0, lmbda=1, a=1, b=1) | Log of the probability density function.
  mean(mu=0, lmbda=1, a=1, b=1) | Distribution mean.
  var(mu=0, lmbda=1, a=1, b=1) | Distribution variance.
  rvs(mu=0, lmbda=1, a=1, b=1, size=None, random_state=None) | Draw random samples.