# -*- coding: utf-8 -*-
"""
Created on Sat Oct 01 13:16:49 2011
Author: Josef Perktold
License: BSD-3
pvalues for Lilliefors test are based on formula and table in
An Analytic Approximation to the Distribution of Lilliefors's Test Statistic for Normality
Author(s): Gerard E. Dallal and Leland WilkinsonSource: The American Statistician, Vol. 40, No. 4 (Nov., 1986), pp. 294-296Published by: American Statistical AssociationStable URL: http://www.jstor.org/stable/2684607 .
On the Kolmogorov-Smirnov Test for Normality with Mean and Variance
Unknown
Hubert W. Lilliefors
Journal of the American Statistical Association, Vol. 62, No. 318. (Jun., 1967), pp. 399-402.
"""
from statsmodels.compat.python import string_types
import numpy as np
from scipy.interpolate import interp1d
from scipy import stats
def ksstat(x, cdf, alternative='two_sided', args=()):
"""
Calculate statistic for the Kolmogorov-Smirnov test for goodness of fit
This calculates the test statistic for a test of the distribution G(x) of an observed
variable against a given distribution F(x). Under the null
hypothesis the two distributions are identical, G(x)=F(x). The
alternative hypothesis can be either 'two_sided' (default), 'less'
or 'greater'. The KS test is only valid for continuous distributions.
Parameters
----------
x : array_like, 1d
array of observations
cdf : string or callable
string: name of a distribution in scipy.stats
callable: function to evaluate cdf
alternative : 'two_sided' (default), 'less' or 'greater'
defines the alternative hypothesis (see explanation)
args : tuple, sequence
distribution parameters for call to cdf
Returns
-------
D : float
KS test statistic, either D, D+ or D-
See Also
--------
scipy.stats.kstest
Notes
-----
In the one-sided test, the alternative is that the empirical
cumulative distribution function of the random variable is "less"
or "greater" than the cumulative distribution function F(x) of the
hypothesis, G(x)<=F(x), resp. G(x)>=F(x).
In contrast to scipy.stats.kstest, this function only calculates the
statistic which can be used either as distance measure or to implement
case specific p-values.
"""
nobs = float(len(x))
if isinstance(cdf, string_types):
cdf = getattr(stats.distributions, cdf).cdf
elif hasattr(cdf, 'cdf'):
cdf = getattr(cdf, 'cdf')
x = np.sort(x)
cdfvals = cdf(x, *args)
if alternative in ['two_sided', 'greater']:
Dplus = (np.arange(1.0, nobs+1)/nobs - cdfvals).max()
if alternative == 'greater':
return Dplus
if alternative in ['two_sided', 'less']:
Dmin = (cdfvals - np.arange(0.0, nobs)/nobs).max()
if alternative == 'less':
return Dmin
D = np.max([Dplus,Dmin])
return D
#new version with tabledist
#--------------------------
def get_lilliefors_table():
#function just to keep things together
from .tabledist import TableDist
#for this test alpha is sf probability, i.e. right tail probability
alpha = np.array([ 0.2 , 0.15 , 0.1 , 0.05 , 0.01 , 0.001])[::-1]
size = np.array([ 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
16, 17, 18, 19, 20, 25, 30, 40, 100, 400, 900], float)
#critical values, rows are by sample size, columns are by alpha
crit_lf = np.array( [[303, 321, 346, 376, 413, 433],
[289, 303, 319, 343, 397, 439],
[269, 281, 297, 323, 371, 424],
[252, 264, 280, 304, 351, 402],
[239, 250, 265, 288, 333, 384],
[227, 238, 252, 274, 317, 365],
[217, 228, 241, 262, 304, 352],
[208, 218, 231, 251, 291, 338],
[200, 210, 222, 242, 281, 325],
[193, 202, 215, 234, 271, 314],
[187, 196, 208, 226, 262, 305],
[181, 190, 201, 219, 254, 296],
[176, 184, 195, 213, 247, 287],
[171, 179, 190, 207, 240, 279],
[167, 175, 185, 202, 234, 273],
[163, 170, 181, 197, 228, 266],
[159, 166, 176, 192, 223, 260],
[143, 150, 159, 173, 201, 236],
[131, 138, 146, 159, 185, 217],
[115, 120, 128, 139, 162, 189],
[ 74, 77, 82, 89, 104, 122],
[ 37, 39, 41, 45, 52, 61],
[ 25, 26, 28, 30, 35, 42]])[:,::-1] / 1000.
lf = TableDist(alpha, size, crit_lf)
return lf
lillifors_table = get_lilliefors_table()
def pval_lf(Dmax, n):
'''approximate pvalues for Lilliefors test of normality
This is only valid for pvalues smaller than 0.1 which is not checked in
this function.
Parameters
----------
Dmax : array_like
two-sided Kolmogorov-Smirnov test statistic
n : int or float
sample size
Returns
-------
p-value : float or ndarray
pvalue according to approximation formula of Dallal and Wilkinson.
Notes
-----
This is mainly a helper function where the calling code should dispatch
on bound violations. Therefore it doesn't check whether the pvalue is in
the valid range.
Precision for the pvalues is around 2 to 3 decimals. This approximation is
also used by other statistical packages (e.g. R:fBasics) but might not be
the most precise available.
References
----------
DallalWilkinson1986
'''
#todo: check boundaries, valid range for n and Dmax
if n>100:
Dmax *= (n/100.)**0.49
n = 100
pval = np.exp(-7.01256*Dmax**2 *(n + 2.78019)
+ 2.99587 * Dmax * np.sqrt(n + 2.78019) - 0.122119
+ 0.974598/np.sqrt(n) + 1.67997/n)
return pval
[docs]def kstest_normal(x, pvalmethod='approx'):
'''Lillifors test for normality,
Kolmogorov Smirnov test with estimated mean and variance
Parameters
----------
x : array_like, 1d
data series, sample
pvalmethod : 'approx', 'table'
'approx' uses the approximation formula of Dalal and Wilkinson,
valid for pvalues < 0.1. If the pvalue is larger than 0.1, then the
result of `table` is returned
'table' uses the table from Dalal and Wilkinson, which is available
for pvalues between 0.001 and 0.2, and the formula of Lilliefors for
large n (n>900). Values in the table are linearly interpolated.
Values outside the range will be returned as bounds, 0.2 for large and
0.001 for small pvalues.
Returns
-------
ksstat : float
Kolmogorov-Smirnov test statistic with estimated mean and variance.
pvalue : float
If the pvalue is lower than some threshold, e.g. 0.05, then we can
reject the Null hypothesis that the sample comes from a normal
distribution
Notes
-----
Reported power to distinguish normal from some other distributions is lower
than with the Anderson-Darling test.
could be vectorized
'''
x = np.asarray(x)
z = (x-x.mean())/x.std(ddof=1)
nobs = len(z)
d_ks = ksstat(z, stats.norm.cdf, alternative='two_sided')
if pvalmethod == 'approx':
pval = pval_lf(d_ks, nobs)
elif pvalmethod == 'table':
#pval = pval_lftable(d_ks, nobs)
pval = lillifors_table.prob(d_ks, nobs)
return d_ks, pval
lillifors = kstest_normal
#old version:
#------------
tble = '''\
00 20 15 10 05 01 .1
4 .303 .321 .346 .376 .413 .433
5 .289 .303 .319 .343 .397 .439
6 .269 .281 .297 .323 .371 .424
7 .252 .264 .280 .304 .351 .402
8 .239 .250 .265 .288 .333 .384
9 .227 .238 .252 .274 .317 .365
10 .217 .228 .241 .262 .304 .352
11 .208 .218 .231 .251 .291 .338
12 .200 .210 .222 .242 .281 .325
13 .193 .202 .215 .234 .271 .314
14 .187 .196 .208 .226 .262 .305
15 .181 .190 .201 .219 .254 .296
16 .176 .184 .195 .213 .247 .287
17 .171 .179 .190 .207 .240 .279
18 .167 .175 .185 .202 .234 .273
19 .163 .170 .181 .197 .228 .266
20 .159 .166 .176 .192 .223 .260
25 .143 .150 .159 .173 .201 .236
30 .131 .138 .146 .159 .185 .217
40 .115 .120 .128 .139 .162 .189
100 .074 .077 .082 .089 .104 .122
400 .037 .039 .041 .045 .052 .061
900 .025 .026 .028 .030 .035 .042'''
'''
parr = np.array([line.split() for line in tble.split('\n')],float)
size = parr[1:,0]
alpha = parr[0,1:] / 100.
crit = parr[1:, 1:]
alpha = np.array([ 0.2 , 0.15 , 0.1 , 0.05 , 0.01 , 0.001])
size = np.array([ 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
16, 17, 18, 19, 20, 25, 30, 40, 100, 400, 900], float)
#critical values, rows are by sample size, columns are by alpha
crit_lf = np.array( [[303, 321, 346, 376, 413, 433],
[289, 303, 319, 343, 397, 439],
[269, 281, 297, 323, 371, 424],
[252, 264, 280, 304, 351, 402],
[239, 250, 265, 288, 333, 384],
[227, 238, 252, 274, 317, 365],
[217, 228, 241, 262, 304, 352],
[208, 218, 231, 251, 291, 338],
[200, 210, 222, 242, 281, 325],
[193, 202, 215, 234, 271, 314],
[187, 196, 208, 226, 262, 305],
[181, 190, 201, 219, 254, 296],
[176, 184, 195, 213, 247, 287],
[171, 179, 190, 207, 240, 279],
[167, 175, 185, 202, 234, 273],
[163, 170, 181, 197, 228, 266],
[159, 166, 176, 192, 223, 260],
[143, 150, 159, 173, 201, 236],
[131, 138, 146, 159, 185, 217],
[115, 120, 128, 139, 162, 189],
[ 74, 77, 82, 89, 104, 122],
[ 37, 39, 41, 45, 52, 61],
[ 25, 26, 28, 30, 35, 42]]) / 1000.
#original Lilliefors paper
crit_greater30 = lambda n: np.array([0.736, 0.768, 0.805, 0.886, 1.031])/np.sqrt(n)
alpha_greater30 = np.array([ 0.2 , 0.15 , 0.1 , 0.05 , 0.01 , 0.001])
n_alpha = 6
polyn = [interp1d(size, crit[:,i]) for i in range(n_alpha)]
def critpolys(n):
return np.array([p(n) for p in polyn])
def pval_lftable(x, n):
#returns extrem probabilities, 0.001 and 0.2, for out of range
critvals = critpolys(n)
if x < critvals[0]:
return alpha[0]
elif x > critvals[-1]:
return alpha[-1]
else:
return interp1d(critvals, alpha)(x)
for n in [19, 19.5, 20, 21, 25]:
print critpolys(n)
print pval_lftable(0.166, 20)
print pval_lftable(0.166, 21)
print 'n=25:', '.103 .052 .010'
print [pval_lf(x, 25) for x in [.159, .173, .201, .236]]
print 'n=10', '.103 .050 .009'
print [pval_lf(x, 10) for x in [.241, .262, .304, .352]]
print 'n=400', '.104 .050 .011'
print [pval_lf(x, 400) for x in crit[-2,2:-1]]
print 'n=900', '.093 .054 .011'
print [pval_lf(x, 900) for x in crit[-1,2:-1]]
print [pval_lftable(x, 400) for x in crit[-2,:]]
print [pval_lftable(x, 300) for x in crit[-3,:]]
xx = np.random.randn(40)
print kstest_normal(xx)
xx2 = np.array([ 1.138, -0.325, -1.461, -0.441, -0.005, -0.957, -1.52 , 0.481,
0.713, 0.175, -1.764, -0.209, -0.681, 0.671, 0.204, 0.403,
-0.165, 1.765, 0.127, -1.261, -0.101, 0.527, 1.114, -0.57 ,
-1.172, 0.697, 0.146, 0.704, 0.422, 0.63 , 0.661, 0.025,
0.177, 0.578, 0.945, 0.211, 0.153, 0.279, 0.35 , 0.396])
( 1.138, -0.325, -1.461, -0.441, -0.005, -0.957, -1.52 , 0.481,
0.713, 0.175, -1.764, -0.209, -0.681, 0.671, 0.204, 0.403,
-0.165, 1.765, 0.127, -1.261, -0.101, 0.527, 1.114, -0.57 ,
-1.172, 0.697, 0.146, 0.704, 0.422, 0.63 , 0.661, 0.025,
0.177, 0.578, 0.945, 0.211, 0.153, 0.279, 0.35 , 0.396)
r_lillieTest = [0.15096827429598147, 0.02225473302348436]
print kstest_normal(xx2), np.array(kstest_normal(xx2)) - r_lillieTest
print kstest_normal(xx2, 'table')
'''
