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def kstest(rvs, cdf, args=(), N=20, alternative='two-sided', method='auto', *, axis=0, nan_policy='propagate', keepdims=False)
Performs the (one-sample or two-sample) Kolmogorov-Smirnov test for
goodness of fit.
The one-sample test compares the underlying distribution F(x) of a sample
against a given distribution G(x). The two-sample test compares the
underlying distributions of two independent samples. Both tests are valid
only for continuous distributions.
Parameters
----------
rvs : str, array_like, or callable
If an array, it should be a 1-D array of observations of random
variables.
If a callable, it should be a function to generate random variables;
it is required to have a keyword argument `size`.
If a string, it should be the name of a distribution in `scipy.stats`,
which will be used to generate random variables.
cdf : str, array_like or callable
If array_like, it should be a 1-D array of observations of random
variables, and the two-sample test is performed
(and rvs must be array_like).
If a callable, that callable is used to calculate the cdf.
If a string, it should be the name of a distribution in `scipy.stats`,
which will be used as the cdf function.
args : tuple, sequence, optional
Distribution parameters, used if `rvs` or `cdf` are strings or
callables.
N : int, optional
Sample size if `rvs` is string or callable. Default is 20.
alternative : {'two-sided', 'less', 'greater'}, optional
Defines the null and alternative hypotheses. Default is 'two-sided'.
Please see explanations in the Notes below.
method : {'auto', 'exact', 'approx', 'asymp'}, optional
Defines the distribution used for calculating the p-value.
The following options are available (default is 'auto'):
* 'auto' : selects one of the other options.
* 'exact' : uses the exact distribution of test statistic.
* 'approx' : approximates the two-sided probability with twice the
one-sided probability
* 'asymp': uses asymptotic distribution of test statistic
axis : int or None, default: 0
If an int, the axis of the input along which to compute the statistic.
The statistic of each axis-slice (e.g. row) of the input will appear in a
corresponding element of the output.
If ``None``, the input will be raveled before computing the statistic.
nan_policy : {'propagate', 'omit', 'raise'}
Defines how to handle input NaNs.
- ``propagate``: if a NaN is present in the axis slice (e.g. row) along
which the statistic is computed, the corresponding entry of the output
will be NaN.
- ``omit``: NaNs will be omitted when performing the calculation.
If insufficient data remains in the axis slice along which the
statistic is computed, the corresponding entry of the output will be
NaN.
- ``raise``: if a NaN is present, a ``ValueError`` will be raised.
keepdims : bool, default: False
If this is set to True, the axes which are reduced are left
in the result as dimensions with size one. With this option,
the result will broadcast correctly against the input array.
Returns
-------
res: KstestResult
An object containing attributes:
statistic : float
KS test statistic, either D+, D-, or D (the maximum of the two)
pvalue : float
One-tailed or two-tailed p-value.
statistic_location : float
In a one-sample test, this is the value of `rvs`
corresponding with the KS statistic; i.e., the distance between
the empirical distribution function and the hypothesized cumulative
distribution function is measured at this observation.
In a two-sample test, this is the value from `rvs` or `cdf`
corresponding with the KS statistic; i.e., the distance between
the empirical distribution functions is measured at this
observation.
statistic_sign : int
In a one-sample test, this is +1 if the KS statistic is the
maximum positive difference between the empirical distribution
function and the hypothesized cumulative distribution function
(D+); it is -1 if the KS statistic is the maximum negative
difference (D-).
In a two-sample test, this is +1 if the empirical distribution
function of `rvs` exceeds the empirical distribution
function of `cdf` at `statistic_location`, otherwise -1.
See Also
--------
:func:`ks_1samp`, :func:`ks_2samp`
..
Notes
-----
There are three options for the null and corresponding alternative
hypothesis that can be selected using the `alternative` parameter.
- `two-sided`: The null hypothesis is that the two distributions are
identical, F(x)=G(x) for all x; the alternative is that they are not
identical.
- `less`: The null hypothesis is that F(x) >= G(x) for all x; the
alternative is that F(x) < G(x) for at least one x.
- `greater`: The null hypothesis is that F(x) <= G(x) for all x; the
alternative is that F(x) > G(x) for at least one x.
Note that the alternative hypotheses describe the *CDFs* of the
underlying distributions, not the observed values. For example,
suppose x1 ~ F and x2 ~ G. If F(x) > G(x) for all x, the values in
x1 tend to be less than those in x2.
Beginning in SciPy 1.9, ``np.matrix`` inputs (not recommended for new
code) are converted to ``np.ndarray`` before the calculation is performed. In
this case, the output will be a scalar or ``np.ndarray`` of appropriate shape
rather than a 2D ``np.matrix``. Similarly, while masked elements of masked
arrays are ignored, the output will be a scalar or ``np.ndarray`` rather than a
masked array with ``mask=False``.
Examples
--------
Suppose we wish to test the null hypothesis that a sample is distributed
according to the standard normal.
We choose a confidence level of 95%; that is, we will reject the null
hypothesis in favor of the alternative if the p-value is less than 0.05.
When testing uniformly distributed data, we would expect the
null hypothesis to be rejected.
>>> import numpy as np
>>> from scipy import stats
>>> rng = np.random.default_rng()
>>> stats.kstest(stats.uniform.rvs(size=100, random_state=rng),
... stats.norm.cdf)
KstestResult(statistic=0.5001899973268688,
pvalue=1.1616392184763533e-23,
statistic_location=0.00047625268963724654,
statistic_sign=-1)
Indeed, the p-value is lower than our threshold of 0.05, so we reject the
null hypothesis in favor of the default "two-sided" alternative: the data
are *not* distributed according to the standard normal.
When testing random variates from the standard normal distribution, we
expect the data to be consistent with the null hypothesis most of the time.
>>> x = stats.norm.rvs(size=100, random_state=rng)
>>> stats.kstest(x, stats.norm.cdf)
KstestResult(statistic=0.05345882212970396,
pvalue=0.9227159037744717,
statistic_location=-1.2451343873745018,
statistic_sign=1)
As expected, the p-value of 0.92 is not below our threshold of 0.05, so
we cannot reject the null hypothesis.
Suppose, however, that the random variates are distributed according to
a normal distribution that is shifted toward greater values. In this case,
the cumulative density function (CDF) of the underlying distribution tends
to be *less* than the CDF of the standard normal. Therefore, we would
expect the null hypothesis to be rejected with ``alternative='less'``:
>>> x = stats.norm.rvs(size=100, loc=0.5, random_state=rng)
>>> stats.kstest(x, stats.norm.cdf, alternative='less')
KstestResult(statistic=0.17482387821055168,
pvalue=0.001913921057766743,
statistic_location=0.3713830565352756,
statistic_sign=-1)
and indeed, with p-value smaller than our threshold, we reject the null
hypothesis in favor of the alternative.
For convenience, the previous test can be performed using the name of the
distribution as the second argument.
>>> stats.kstest(x, "norm", alternative='less')
KstestResult(statistic=0.17482387821055168,
pvalue=0.001913921057766743,
statistic_location=0.3713830565352756,
statistic_sign=-1)
The examples above have all been one-sample tests identical to those
performed by `ks_1samp`. Note that `kstest` can also perform two-sample
tests identical to those performed by `ks_2samp`. For example, when two
samples are drawn from the same distribution, we expect the data to be
consistent with the null hypothesis most of the time.
>>> sample1 = stats.laplace.rvs(size=105, random_state=rng)
>>> sample2 = stats.laplace.rvs(size=95, random_state=rng)
>>> stats.kstest(sample1, sample2)
KstestResult(statistic=0.11779448621553884,
pvalue=0.4494256912629795,
statistic_location=0.6138814275424155,
statistic_sign=1)
As expected, the p-value of 0.45 is not below our threshold of 0.05, so
we cannot reject the null hypothesis.
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