# Source code for statsmodels.stats.dist_dependence_measures

```
""" Distance dependence measure and the dCov test.
Implementation of SzĂ©kely et al. (2007) calculation of distance
dependence statistics, including the Distance covariance (dCov) test
for independence of random vectors of arbitrary length.
Author: Ron Itzikovitch
References
----------
.. Szekely, G.J., Rizzo, M.L., and Bakirov, N.K. (2007)
"Measuring and testing dependence by correlation of distances".
Annals of Statistics, Vol. 35 No. 6, pp. 2769-2794.
"""
from collections import namedtuple
import warnings
import numpy as np
from scipy.spatial.distance import pdist, squareform
from scipy.stats import norm
from statsmodels.tools.sm_exceptions import HypothesisTestWarning
DistDependStat = namedtuple(
"DistDependStat",
["test_statistic", "distance_correlation",
"distance_covariance", "dvar_x", "dvar_y", "S"],
)
[docs]
def distance_covariance_test(x, y, B=None, method="auto"):
r"""The Distance Covariance (dCov) test
Apply the Distance Covariance (dCov) test of independence to `x` and `y`.
This test was introduced in [1]_, and is based on the distance covariance
statistic. The test is applicable to random vectors of arbitrary length
(see the notes section for more details).
Parameters
----------
x : array_like, 1-D or 2-D
If `x` is 1-D than it is assumed to be a vector of observations of a
single random variable. If `x` is 2-D than the rows should be
observations and the columns are treated as the components of a
random vector, i.e., each column represents a different component of
the random vector `x`.
y : array_like, 1-D or 2-D
Same as `x`, but only the number of observation has to match that of
`x`. If `y` is 2-D note that the number of columns of `y` (i.e., the
number of components in the random vector) does not need to match
the number of columns in `x`.
B : int, optional, default=`None`
The number of iterations to perform when evaluating the null
distribution of the test statistic when the `emp` method is
applied (see below). if `B` is `None` than as in [1]_ we set
`B` to be ``B = 200 + 5000/n``, where `n` is the number of
observations.
method : {'auto', 'emp', 'asym'}, optional, default=auto
The method by which to obtain the p-value for the test.
- `auto` : Default method. The number of observations will be used to
determine the method.
- `emp` : Empirical evaluation of the p-value using permutations of
the rows of `y` to obtain the null distribution.
- `asym` : An asymptotic approximation of the distribution of the test
statistic is used to find the p-value.
Returns
-------
test_statistic : float
The value of the test statistic used in the test.
pval : float
The p-value.
chosen_method : str
The method that was used to obtain the p-value. Mostly relevant when
the function is called with `method='auto'`.
Notes
-----
The test applies to random vectors of arbitrary dimensions, i.e., `x`
can be a 1-D vector of observations for a single random variable while
`y` can be a `k` by `n` 2-D array (where `k > 1`). In other words, it
is also possible for `x` and `y` to both be 2-D arrays and have the
same number of rows (observations) while differing in the number of
columns.
As noted in [1]_ the statistics are sensitive to all types of departures
from independence, including nonlinear or nonmonotone dependence
structure.
References
----------
.. [1] Szekely, G.J., Rizzo, M.L., and Bakirov, N.K. (2007)
"Measuring and testing by correlation of distances".
Annals of Statistics, Vol. 35 No. 6, pp. 2769-2794.
Examples
--------
>>> from statsmodels.stats.dist_dependence_measures import
... distance_covariance_test
>>> data = np.random.rand(1000, 10)
>>> x, y = data[:, :3], data[:, 3:]
>>> x.shape
(1000, 3)
>>> y.shape
(1000, 7)
>>> distance_covariance_test(x, y)
(1.0426404792714983, 0.2971148340813543, 'asym')
# (test_statistic, pval, chosen_method)
"""
x, y = _validate_and_tranform_x_and_y(x, y)
n = x.shape[0]
stats = distance_statistics(x, y)
if method == "auto" and n <= 500 or method == "emp":
chosen_method = "emp"
test_statistic, pval = _empirical_pvalue(x, y, B, n, stats)
elif method == "auto" and n > 500 or method == "asym":
chosen_method = "asym"
test_statistic, pval = _asymptotic_pvalue(stats)
else:
raise ValueError(f"Unknown 'method' parameter: {method}")
# In case we got an extreme p-value (0 or 1) when using the empirical
# distribution of the test statistic under the null, we fall back
# to the asymptotic approximation.
if chosen_method == "emp" and pval in [0, 1]:
msg = (
f"p-value was {pval} when using the empirical method. "
"The asymptotic approximation will be used instead"
)
warnings.warn(msg, HypothesisTestWarning)
_, pval = _asymptotic_pvalue(stats)
return test_statistic, pval, chosen_method
def _validate_and_tranform_x_and_y(x, y):
r"""Ensure `x` and `y` have proper shape and transform/reshape them if
required.
Parameters
----------
x : array_like, 1-D or 2-D
If `x` is 1-D than it is assumed to be a vector of observations of a
single random variable. If `x` is 2-D than the rows should be
observations and the columns are treated as the components of a
random vector, i.e., each column represents a different component of
the random vector `x`.
y : array_like, 1-D or 2-D
Same as `x`, but only the number of observation has to match that of
`x`. If `y` is 2-D note that the number of columns of `y` (i.e., the
number of components in the random vector) does not need to match
the number of columns in `x`.
Returns
-------
x : array_like, 1-D or 2-D
y : array_like, 1-D or 2-D
Raises
------
ValueError
If `x` and `y` have a different number of observations.
"""
x = np.asanyarray(x)
y = np.asanyarray(y)
if x.shape[0] != y.shape[0]:
raise ValueError(
"x and y must have the same number of observations (rows)."
)
if len(x.shape) == 1:
x = x.reshape((x.shape[0], 1))
if len(y.shape) == 1:
y = y.reshape((y.shape[0], 1))
return x, y
def _empirical_pvalue(x, y, B, n, stats):
r"""Calculate the empirical p-value based on permutations of `y`'s rows
Parameters
----------
x : array_like, 1-D or 2-D
If `x` is 1-D than it is assumed to be a vector of observations of a
single random variable. If `x` is 2-D than the rows should be
observations and the columns are treated as the components of a
random vector, i.e., each column represents a different component of
the random vector `x`.
y : array_like, 1-D or 2-D
Same as `x`, but only the number of observation has to match that of
`x`. If `y` is 2-D note that the number of columns of `y` (i.e., the
number of components in the random vector) does not need to match
the number of columns in `x`.
B : int
The number of iterations when evaluating the null distribution.
n : Number of observations found in each of `x` and `y`.
stats: namedtuple
The result obtained from calling ``distance_statistics(x, y)``.
Returns
-------
test_statistic : float
The empirical test statistic.
pval : float
The empirical p-value.
"""
B = int(B) if B else int(np.floor(200 + 5000 / n))
empirical_dist = _get_test_statistic_distribution(x, y, B)
pval = 1 - np.searchsorted(
sorted(empirical_dist), stats.test_statistic
) / len(empirical_dist)
test_statistic = stats.test_statistic
return test_statistic, pval
def _asymptotic_pvalue(stats):
r"""Calculate the p-value based on an approximation of the distribution of
the test statistic under the null.
Parameters
----------
stats: namedtuple
The result obtained from calling ``distance_statistics(x, y)``.
Returns
-------
test_statistic : float
The test statistic.
pval : float
The asymptotic p-value.
"""
test_statistic = np.sqrt(stats.test_statistic / stats.S)
pval = (1 - norm.cdf(test_statistic)) * 2
return test_statistic, pval
def _get_test_statistic_distribution(x, y, B):
r"""
Parameters
----------
x : array_like, 1-D or 2-D
If `x` is 1-D than it is assumed to be a vector of observations of a
single random variable. If `x` is 2-D than the rows should be
observations and the columns are treated as the components of a
random vector, i.e., each column represents a different component of
the random vector `x`.
y : array_like, 1-D or 2-D
Same as `x`, but only the number of observation has to match that of
`x`. If `y` is 2-D note that the number of columns of `y` (i.e., the
number of components in the random vector) does not need to match
the number of columns in `x`.
B : int
The number of iterations to perform when evaluating the null
distribution.
Returns
-------
emp_dist : array_like
The empirical distribution of the test statistic.
"""
y = y.copy()
emp_dist = np.zeros(B)
x_dist = squareform(pdist(x, "euclidean"))
for i in range(B):
np.random.shuffle(y)
emp_dist[i] = distance_statistics(x, y, x_dist=x_dist).test_statistic
return emp_dist
[docs]
def distance_statistics(x, y, x_dist=None, y_dist=None):
r"""Calculate various distance dependence statistics.
Calculate several distance dependence statistics as described in [1]_.
Parameters
----------
x : array_like, 1-D or 2-D
If `x` is 1-D than it is assumed to be a vector of observations of a
single random variable. If `x` is 2-D than the rows should be
observations and the columns are treated as the components of a
random vector, i.e., each column represents a different component of
the random vector `x`.
y : array_like, 1-D or 2-D
Same as `x`, but only the number of observation has to match that of
`x`. If `y` is 2-D note that the number of columns of `y` (i.e., the
number of components in the random vector) does not need to match
the number of columns in `x`.
x_dist : array_like, 2-D, optional
A square 2-D array_like object whose values are the euclidean
distances between `x`'s rows.
y_dist : array_like, 2-D, optional
A square 2-D array_like object whose values are the euclidean
distances between `y`'s rows.
Returns
-------
namedtuple
A named tuple of distance dependence statistics (DistDependStat) with
the following values:
- test_statistic : float - The "basic" test statistic (i.e., the one
used when the `emp` method is chosen when calling
``distance_covariance_test()``
- distance_correlation : float - The distance correlation
between `x` and `y`.
- distance_covariance : float - The distance covariance of
`x` and `y`.
- dvar_x : float - The distance variance of `x`.
- dvar_y : float - The distance variance of `y`.
- S : float - The mean of the euclidean distances in `x` multiplied
by those of `y`. Mostly used internally.
References
----------
.. [1] Szekely, G.J., Rizzo, M.L., and Bakirov, N.K. (2007)
"Measuring and testing dependence by correlation of distances".
Annals of Statistics, Vol. 35 No. 6, pp. 2769-2794.
Examples
--------
>>> from statsmodels.stats.dist_dependence_measures import
... distance_statistics
>>> distance_statistics(np.random.random(1000), np.random.random(1000))
DistDependStat(test_statistic=0.07948284320205831,
distance_correlation=0.04269511890990793,
distance_covariance=0.008915315092696293,
dvar_x=0.20719027438266704, dvar_y=0.21044934264957588,
S=0.10892061635588891)
"""
x, y = _validate_and_tranform_x_and_y(x, y)
n = x.shape[0]
a = x_dist if x_dist is not None else squareform(pdist(x, "euclidean"))
b = y_dist if y_dist is not None else squareform(pdist(y, "euclidean"))
a_row_means = a.mean(axis=0, keepdims=True)
b_row_means = b.mean(axis=0, keepdims=True)
a_col_means = a.mean(axis=1, keepdims=True)
b_col_means = b.mean(axis=1, keepdims=True)
a_mean = a.mean()
b_mean = b.mean()
A = a - a_row_means - a_col_means + a_mean
B = b - b_row_means - b_col_means + b_mean
S = a_mean * b_mean
dcov = np.sqrt(np.multiply(A, B).mean())
dvar_x = np.sqrt(np.multiply(A, A).mean())
dvar_y = np.sqrt(np.multiply(B, B).mean())
dcor = dcov / np.sqrt(dvar_x * dvar_y)
test_statistic = n * dcov ** 2
return DistDependStat(
test_statistic=test_statistic,
distance_correlation=dcor,
distance_covariance=dcov,
dvar_x=dvar_x,
dvar_y=dvar_y,
S=S,
)
[docs]
def distance_covariance(x, y):
r"""Distance covariance.
Calculate the empirical distance covariance as described in [1]_.
Parameters
----------
x : array_like, 1-D or 2-D
If `x` is 1-D than it is assumed to be a vector of observations of a
single random variable. If `x` is 2-D than the rows should be
observations and the columns are treated as the components of a
random vector, i.e., each column represents a different component of
the random vector `x`.
y : array_like, 1-D or 2-D
Same as `x`, but only the number of observation has to match that of
`x`. If `y` is 2-D note that the number of columns of `y` (i.e., the
number of components in the random vector) does not need to match
the number of columns in `x`.
Returns
-------
float
The empirical distance covariance between `x` and `y`.
References
----------
.. [1] Szekely, G.J., Rizzo, M.L., and Bakirov, N.K. (2007)
"Measuring and testing dependence by correlation of distances".
Annals of Statistics, Vol. 35 No. 6, pp. 2769-2794.
Examples
--------
>>> from statsmodels.stats.dist_dependence_measures import
... distance_covariance
>>> distance_covariance(np.random.random(1000), np.random.random(1000))
0.007575063951951362
"""
return distance_statistics(x, y).distance_covariance
[docs]
def distance_variance(x):
r"""Distance variance.
Calculate the empirical distance variance as described in [1]_.
Parameters
----------
x : array_like, 1-D or 2-D
If `x` is 1-D than it is assumed to be a vector of observations of a
single random variable. If `x` is 2-D than the rows should be
observations and the columns are treated as the components of a
random vector, i.e., each column represents a different component of
the random vector `x`.
Returns
-------
float
The empirical distance variance of `x`.
References
----------
.. [1] Szekely, G.J., Rizzo, M.L., and Bakirov, N.K. (2007)
"Measuring and testing dependence by correlation of distances".
Annals of Statistics, Vol. 35 No. 6, pp. 2769-2794.
Examples
--------
>>> from statsmodels.stats.dist_dependence_measures import
... distance_variance
>>> distance_variance(np.random.random(1000))
0.21732609190659702
"""
return distance_covariance(x, x)
[docs]
def distance_correlation(x, y):
r"""Distance correlation.
Calculate the empirical distance correlation as described in [1]_.
This statistic is analogous to product-moment correlation and describes
the dependence between `x` and `y`, which are random vectors of
arbitrary length. The statistics' values range between 0 (implies
independence) and 1 (implies complete dependence).
Parameters
----------
x : array_like, 1-D or 2-D
If `x` is 1-D than it is assumed to be a vector of observations of a
single random variable. If `x` is 2-D than the rows should be
observations and the columns are treated as the components of a
random vector, i.e., each column represents a different component of
the random vector `x`.
y : array_like, 1-D or 2-D
Same as `x`, but only the number of observation has to match that of
`x`. If `y` is 2-D note that the number of columns of `y` (i.e., the
number of components in the random vector) does not need to match
the number of columns in `x`.
Returns
-------
float
The empirical distance correlation between `x` and `y`.
References
----------
.. [1] Szekely, G.J., Rizzo, M.L., and Bakirov, N.K. (2007)
"Measuring and testing dependence by correlation of distances".
Annals of Statistics, Vol. 35 No. 6, pp. 2769-2794.
Examples
--------
>>> from statsmodels.stats.dist_dependence_measures import
... distance_correlation
>>> distance_correlation(np.random.random(1000), np.random.random(1000))
0.04060497840149489
"""
return distance_statistics(x, y).distance_correlation
```

Last update:
Jul 16, 2024