scipy.stats.gamma¶

scipy.stats.gamma

A gamma continuous random variable.

Continuous random variables are defined from a standard form and may require some shape parameters to complete its specification. Any optional keyword parameters can be passed to the methods of the RV object as given below:

Parameters : x : array-like quantiles q : array-like lower or upper tail probability a : array-like shape parameters loc : array-like, optional location parameter (default=0) scale : array-like, optional scale parameter (default=1) size : int or tuple of ints, optional shape of random variates (default computed from input arguments ) moments : str, optional composed of letters [‘mvsk’] specifying which moments to compute where ‘m’ = mean, ‘v’ = variance, ‘s’ = (Fisher’s) skew and ‘k’ = (Fisher’s) kurtosis. (default=’mv’) Alternatively, the object may be called (as a function) to fix the shape, : location, and scale parameters returning a “frozen” continuous RV object: : rv = gamma(a, loc=0, scale=1) : Frozen RV object with the same methods but holding the given shape, location, and scale fixed.

Notes

Gamma distribution

For a = integer, this is the Erlang distribution, and for a=1 it is the exponential distribution.

gamma.pdf(x,a) = x**(a-1)*exp(-x)/gamma(a) for x >= 0, a > 0.

Examples

```>>> import matplotlib.pyplot as plt
>>> numargs = gamma.numargs
>>> [ a ] = [0.9,] * numargs
>>> rv = gamma(a)
```

Display frozen pdf

```>>> x = np.linspace(0, np.minimum(rv.dist.b, 3))
>>> h = plt.plot(x, rv.pdf(x))
```

Check accuracy of cdf and ppf

```>>> prb = gamma.cdf(x, a)
>>> h = plt.semilogy(np.abs(x - gamma.ppf(prb, a)) + 1e-20)
```

Random number generation

```>>> R = gamma.rvs(a, size=100)
```

Methods

 rvs(a, loc=0, scale=1, size=1) Random variates. pdf(x, a, loc=0, scale=1) Probability density function. cdf(x, a, loc=0, scale=1) Cumulative density function. sf(x, a, loc=0, scale=1) Survival function (1-cdf — sometimes more accurate). ppf(q, a, loc=0, scale=1) Percent point function (inverse of cdf — percentiles). isf(q, a, loc=0, scale=1) Inverse survival function (inverse of sf). stats(a, loc=0, scale=1, moments=’mv’) Mean(‘m’), variance(‘v’), skew(‘s’), and/or kurtosis(‘k’). entropy(a, loc=0, scale=1) (Differential) entropy of the RV. fit(data, a, loc=0, scale=1) Parameter estimates for generic data.

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