scipy.special.smirnov#

scipy.special.smirnov(n, d, out=None) = <wrapped_ufunc 'smirnov'>#

Kolmogorov-Smirnov complementary cumulative distribution function.

Returns the exact Kolmogorov-Smirnov complementary cumulative distribution function,(aka the Survival Function) of Dn+ (or Dn-) for a one-sided test of equality between an empirical and a theoretical distribution. It is equal to the probability that the maximum difference between a theoretical distribution and an empirical one based on n samples is greater than d.

Parameters:

nint: Number of samples
dfloat array_like: Deviation between the Empirical CDF (ECDF) and the target CDF.
outndarray, optional: Optional output array for the function results

Returns:

scalar or ndarray: The value(s) of smirnov(n, d), Prob(Dn+ >= d) (Also Prob(Dn- >= d))

See also

smirnovi: The Inverse Survival Function for the distribution
scipy.stats.ksone: Provides the functionality as a continuous distribution
kolmogorov, kolmogi: Functions for the two-sided distribution

Notes

smirnov is used by scipy.stats.kstest in the application of the Kolmogorov-Smirnov Goodness of Fit test. For historical reasons this function is exposed in scipy.special, but the recommended way to achieve the most accurate CDF/SF/PDF/PPF/ISF computations is to use the scipy.stats.ksone distribution.

Array API Standard Support

smirnov has support for Python Array API Standard compatible backends in addition to NumPy. The following combinations of backend and device (or other capability) are supported.

Library	CPU	GPU
NumPy	✅	n/a
CuPy	n/a	⛔
PyTorch	✅	⛔
JAX	✅	⛔
Dask	✅	n/a

For the NumPy backend, this function supports all NumPy ufunc keyword arguments. Other backends may support out, but typically none of the other ufunc kwargs. out is typically supported for CuPy and PyTorch, but not currently in cases where SciPy relies on a generic Array API implementation or, for PyTorch on CPU, falls back to the NumPy backend. out is never supported for JAX because JAX arrays are immutable. smirnov does not currently support out for the PyTorch backend.

See Support for the array API standard for more information.

Examples

>>> import numpy as np
>>> from scipy.special import smirnov
>>> from scipy.stats import norm

Show the probability of a gap at least as big as 0, 0.5 and 1.0 for a sample of size 5.

>>> smirnov(5, [0, 0.5, 1.0])
array([ 1.   ,  0.056,  0.   ])

Compare a sample of size 5 against N(0, 1), the standard normal distribution with mean 0 and standard deviation 1.

x is the sample.

>>> x = np.array([-1.392, -0.135, 0.114, 0.190, 1.82])

>>> target = norm(0, 1)
>>> cdfs = target.cdf(x)
>>> cdfs
array([0.0819612 , 0.44630594, 0.5453811 , 0.57534543, 0.9656205 ])

Construct the empirical CDF and the K-S statistics (Dn+, Dn-, Dn).

>>> n = len(x)
>>> ecdfs = np.arange(n+1, dtype=float)/n
>>> cols = np.column_stack([x, ecdfs[1:], cdfs, cdfs - ecdfs[:n],
...                        ecdfs[1:] - cdfs])
>>> with np.printoptions(precision=3):
...    print(cols)
[[-1.392  0.2    0.082  0.082  0.118]
 [-0.135  0.4    0.446  0.246 -0.046]
 [ 0.114  0.6    0.545  0.145  0.055]
 [ 0.19   0.8    0.575 -0.025  0.225]
 [ 1.82   1.     0.966  0.166  0.034]]
>>> gaps = cols[:, -2:]
>>> Dnpm = np.max(gaps, axis=0)
>>> print(f'Dn-={Dnpm[0]:f}, Dn+={Dnpm[1]:f}')
Dn-=0.246306, Dn+=0.224655
>>> probs = smirnov(n, Dnpm)
>>> print(f'For a sample of size {n} drawn from N(0, 1):',
...       f' Smirnov n={n}: Prob(Dn- >= {Dnpm[0]:f}) = {probs[0]:.4f}',
...       f' Smirnov n={n}: Prob(Dn+ >= {Dnpm[1]:f}) = {probs[1]:.4f}',
...       sep='\n')
For a sample of size 5 drawn from N(0, 1):
 Smirnov n=5: Prob(Dn- >= 0.246306) = 0.4711
 Smirnov n=5: Prob(Dn+ >= 0.224655) = 0.5245

Plot the empirical CDF and the standard normal CDF.

>>> import matplotlib.pyplot as plt
>>> plt.step(np.concatenate(([-2.5], x, [2.5])),
...          np.concatenate((ecdfs, [1])),
...          where='post', label='Empirical CDF')
>>> xx = np.linspace(-2.5, 2.5, 100)
>>> plt.plot(xx, target.cdf(xx), '--', label='CDF for N(0, 1)')

Add vertical lines marking Dn+ and Dn-.

>>> iminus, iplus = np.argmax(gaps, axis=0)
>>> plt.vlines([x[iminus]], ecdfs[iminus], cdfs[iminus], color='r',
...            alpha=0.5, lw=4)
>>> plt.vlines([x[iplus]], cdfs[iplus], ecdfs[iplus+1], color='m',
...            alpha=0.5, lw=4)

>>> plt.grid(True)
>>> plt.legend(framealpha=1, shadow=True)
>>> plt.show()

../../_images/scipy-special-smirnov-1.png