"""
Robust location and covariance estimators.
Here are implemented estimators that are resistant to outliers.
"""
# Author: Virgile Fritsch <virgile.fritsch@inria.fr>
#
# License: BSD 3 clause
import warnings
import numbers
import numpy as np
from scipy import linalg
from scipy.stats import chi2
from . import empirical_covariance, EmpiricalCovariance
from ..utils.extmath import fast_logdet
from ..utils import check_random_state, check_array
# Minimum Covariance Determinant
# Implementing of an algorithm by Rousseeuw & Van Driessen described in
# (A Fast Algorithm for the Minimum Covariance Determinant Estimator,
# 1999, American Statistical Association and the American Society
# for Quality, TECHNOMETRICS)
# XXX Is this really a public function? It's not listed in the docs or
# exported by sklearn.covariance. Deprecate?
def c_step(X, n_support, remaining_iterations=30, initial_estimates=None,
verbose=False, cov_computation_method=empirical_covariance,
random_state=None):
"""C_step procedure described in [Rouseeuw1984]_ aiming at computing MCD.
Parameters
----------
X : array-like, shape (n_samples, n_features)
Data set in which we look for the n_support observations whose
scatter matrix has minimum determinant.
n_support : int, > n_samples / 2
Number of observations to compute the robust estimates of location
and covariance from.
remaining_iterations : int, optional
Number of iterations to perform.
According to [Rouseeuw1999]_, two iterations are sufficient to get
close to the minimum, and we never need more than 30 to reach
convergence.
initial_estimates : 2-tuple, optional
Initial estimates of location and shape from which to run the c_step
procedure:
- initial_estimates[0]: an initial location estimate
- initial_estimates[1]: an initial covariance estimate
verbose : boolean, optional
Verbose mode.
random_state : int, RandomState instance or None, optional (default=None)
If int, random_state is the seed used by the random number generator;
If RandomState instance, random_state is the random number generator;
If None, the random number generator is the RandomState instance used
by `np.random`.
cov_computation_method : callable, default empirical_covariance
The function which will be used to compute the covariance.
Must return shape (n_features, n_features)
Returns
-------
location : array-like, shape (n_features,)
Robust location estimates.
covariance : array-like, shape (n_features, n_features)
Robust covariance estimates.
support : array-like, shape (n_samples,)
A mask for the `n_support` observations whose scatter matrix has
minimum determinant.
References
----------
.. [Rouseeuw1999] A Fast Algorithm for the Minimum Covariance Determinant
Estimator, 1999, American Statistical Association and the American
Society for Quality, TECHNOMETRICS
"""
X = np.asarray(X)
random_state = check_random_state(random_state)
return _c_step(X, n_support, remaining_iterations=remaining_iterations,
initial_estimates=initial_estimates, verbose=verbose,
cov_computation_method=cov_computation_method,
random_state=random_state)
def _c_step(X, n_support, random_state, remaining_iterations=30,
initial_estimates=None, verbose=False,
cov_computation_method=empirical_covariance):
n_samples, n_features = X.shape
dist = np.inf
# Initialisation
support = np.zeros(n_samples, dtype=bool)
if initial_estimates is None:
# compute initial robust estimates from a random subset
support[random_state.permutation(n_samples)[:n_support]] = True
else:
# get initial robust estimates from the function parameters
location = initial_estimates[0]
covariance = initial_estimates[1]
# run a special iteration for that case (to get an initial support)
precision = linalg.pinvh(covariance)
X_centered = X - location
dist = (np.dot(X_centered, precision) * X_centered).sum(1)
# compute new estimates
support[np.argsort(dist)[:n_support]] = True
X_support = X[support]
location = X_support.mean(0)
covariance = cov_computation_method(X_support)
# Iterative procedure for Minimum Covariance Determinant computation
det = fast_logdet(covariance)
# If the data already has singular covariance, calculate the precision,
# as the loop below will not be entered.
if np.isinf(det):
precision = linalg.pinvh(covariance)
previous_det = np.inf
while (det < previous_det and remaining_iterations > 0
and not np.isinf(det)):
# save old estimates values
previous_location = location
previous_covariance = covariance
previous_det = det
previous_support = support
# compute a new support from the full data set mahalanobis distances
precision = linalg.pinvh(covariance)
X_centered = X - location
dist = (np.dot(X_centered, precision) * X_centered).sum(axis=1)
# compute new estimates
support = np.zeros(n_samples, dtype=bool)
support[np.argsort(dist)[:n_support]] = True
X_support = X[support]
location = X_support.mean(axis=0)
covariance = cov_computation_method(X_support)
det = fast_logdet(covariance)
# update remaining iterations for early stopping
remaining_iterations -= 1
previous_dist = dist
dist = (np.dot(X - location, precision) * (X - location)).sum(axis=1)
# Check if best fit already found (det => 0, logdet => -inf)
if np.isinf(det):
results = location, covariance, det, support, dist
# Check convergence
if np.allclose(det, previous_det):
# c_step procedure converged
if verbose:
print("Optimal couple (location, covariance) found before"
" ending iterations (%d left)" % (remaining_iterations))
results = location, covariance, det, support, dist
elif det > previous_det:
# determinant has increased (should not happen)
warnings.warn("Warning! det > previous_det (%.15f > %.15f)"
% (det, previous_det), RuntimeWarning)
results = previous_location, previous_covariance, \
previous_det, previous_support, previous_dist
# Check early stopping
if remaining_iterations == 0:
if verbose:
print('Maximum number of iterations reached')
results = location, covariance, det, support, dist
return results
def select_candidates(X, n_support, n_trials, select=1, n_iter=30,
verbose=False,
cov_computation_method=empirical_covariance,
random_state=None):
"""Finds the best pure subset of observations to compute MCD from it.
The purpose of this function is to find the best sets of n_support
observations with respect to a minimization of their covariance
matrix determinant. Equivalently, it removes n_samples-n_support
observations to construct what we call a pure data set (i.e. not
containing outliers). The list of the observations of the pure
data set is referred to as the `support`.
Starting from a random support, the pure data set is found by the
c_step procedure introduced by Rousseeuw and Van Driessen in
[RV]_.
Parameters
----------
X : array-like, shape (n_samples, n_features)
Data (sub)set in which we look for the n_support purest observations.
n_support : int, [(n + p + 1)/2] < n_support < n
The number of samples the pure data set must contain.
select : int, int > 0
Number of best candidates results to return.
n_trials : int, nb_trials > 0 or 2-tuple
Number of different initial sets of observations from which to
run the algorithm.
Instead of giving a number of trials to perform, one can provide a
list of initial estimates that will be used to iteratively run
c_step procedures. In this case:
- n_trials[0]: array-like, shape (n_trials, n_features)
is the list of `n_trials` initial location estimates
- n_trials[1]: array-like, shape (n_trials, n_features, n_features)
is the list of `n_trials` initial covariances estimates
n_iter : int, nb_iter > 0
Maximum number of iterations for the c_step procedure.
(2 is enough to be close to the final solution. "Never" exceeds 20).
random_state : int, RandomState instance or None, optional (default=None)
If int, random_state is the seed used by the random number generator;
If RandomState instance, random_state is the random number generator;
If None, the random number generator is the RandomState instance used
by `np.random`.
cov_computation_method : callable, default empirical_covariance
The function which will be used to compute the covariance.
Must return shape (n_features, n_features)
verbose : boolean, default False
Control the output verbosity.
See Also
---------
c_step
Returns
-------
best_locations : array-like, shape (select, n_features)
The `select` location estimates computed from the `select` best
supports found in the data set (`X`).
best_covariances : array-like, shape (select, n_features, n_features)
The `select` covariance estimates computed from the `select`
best supports found in the data set (`X`).
best_supports : array-like, shape (select, n_samples)
The `select` best supports found in the data set (`X`).
References
----------
.. [RV] A Fast Algorithm for the Minimum Covariance Determinant
Estimator, 1999, American Statistical Association and the American
Society for Quality, TECHNOMETRICS
"""
random_state = check_random_state(random_state)
n_samples, n_features = X.shape
if isinstance(n_trials, numbers.Integral):
run_from_estimates = False
elif isinstance(n_trials, tuple):
run_from_estimates = True
estimates_list = n_trials
n_trials = estimates_list[0].shape[0]
else:
raise TypeError("Invalid 'n_trials' parameter, expected tuple or "
" integer, got %s (%s)" % (n_trials, type(n_trials)))
# compute `n_trials` location and shape estimates candidates in the subset
all_estimates = []
if not run_from_estimates:
# perform `n_trials` computations from random initial supports
for j in range(n_trials):
all_estimates.append(
_c_step(
X, n_support, remaining_iterations=n_iter, verbose=verbose,
cov_computation_method=cov_computation_method,
random_state=random_state))
else:
# perform computations from every given initial estimates
for j in range(n_trials):
initial_estimates = (estimates_list[0][j], estimates_list[1][j])
all_estimates.append(_c_step(
X, n_support, remaining_iterations=n_iter,
initial_estimates=initial_estimates, verbose=verbose,
cov_computation_method=cov_computation_method,
random_state=random_state))
all_locs_sub, all_covs_sub, all_dets_sub, all_supports_sub, all_ds_sub = \
zip(*all_estimates)
# find the `n_best` best results among the `n_trials` ones
index_best = np.argsort(all_dets_sub)[:select]
best_locations = np.asarray(all_locs_sub)[index_best]
best_covariances = np.asarray(all_covs_sub)[index_best]
best_supports = np.asarray(all_supports_sub)[index_best]
best_ds = np.asarray(all_ds_sub)[index_best]
return best_locations, best_covariances, best_supports, best_ds
def fast_mcd(X, support_fraction=None,
cov_computation_method=empirical_covariance,
random_state=None):
"""Estimates the Minimum Covariance Determinant matrix.
Read more in the :ref:`User Guide <robust_covariance>`.
Parameters
----------
X : array-like, shape (n_samples, n_features)
The data matrix, with p features and n samples.
support_fraction : float, 0 < support_fraction < 1
The proportion of points to be included in the support of the raw
MCD estimate. Default is None, which implies that the minimum
value of support_fraction will be used within the algorithm:
`[n_sample + n_features + 1] / 2`.
cov_computation_method : callable, default empirical_covariance
The function which will be used to compute the covariance.
Must return shape (n_features, n_features)
random_state : int, RandomState instance or None, optional (default=None)
If int, random_state is the seed used by the random number generator;
If RandomState instance, random_state is the random number generator;
If None, the random number generator is the RandomState instance used
by `np.random`.
Notes
-----
The FastMCD algorithm has been introduced by Rousseuw and Van Driessen
in "A Fast Algorithm for the Minimum Covariance Determinant Estimator,
1999, American Statistical Association and the American Society
for Quality, TECHNOMETRICS".
The principle is to compute robust estimates and random subsets before
pooling them into a larger subsets, and finally into the full data set.
Depending on the size of the initial sample, we have one, two or three
such computation levels.
Note that only raw estimates are returned. If one is interested in
the correction and reweighting steps described in [RouseeuwVan]_,
see the MinCovDet object.
References
----------
.. [RouseeuwVan] A Fast Algorithm for the Minimum Covariance
Determinant Estimator, 1999, American Statistical Association
and the American Society for Quality, TECHNOMETRICS
.. [Butler1993] R. W. Butler, P. L. Davies and M. Jhun,
Asymptotics For The Minimum Covariance Determinant Estimator,
The Annals of Statistics, 1993, Vol. 21, No. 3, 1385-1400
Returns
-------
location : array-like, shape (n_features,)
Robust location of the data.
covariance : array-like, shape (n_features, n_features)
Robust covariance of the features.
support : array-like, type boolean, shape (n_samples,)
A mask of the observations that have been used to compute
the robust location and covariance estimates of the data set.
"""
random_state = check_random_state(random_state)
X = check_array(X, ensure_min_samples=2, estimator='fast_mcd')
n_samples, n_features = X.shape
# minimum breakdown value
if support_fraction is None:
n_support = int(np.ceil(0.5 * (n_samples + n_features + 1)))
else:
n_support = int(support_fraction * n_samples)
# 1-dimensional case quick computation
# (Rousseeuw, P. J. and Leroy, A. M. (2005) References, in Robust
# Regression and Outlier Detection, John Wiley & Sons, chapter 4)
if n_features == 1:
if n_support < n_samples:
# find the sample shortest halves
X_sorted = np.sort(np.ravel(X))
diff = X_sorted[n_support:] - X_sorted[:(n_samples - n_support)]
halves_start = np.where(diff == np.min(diff))[0]
# take the middle points' mean to get the robust location estimate
location = 0.5 * (X_sorted[n_support + halves_start] +
X_sorted[halves_start]).mean()
support = np.zeros(n_samples, dtype=bool)
X_centered = X - location
support[np.argsort(np.abs(X_centered), 0)[:n_support]] = True
covariance = np.asarray([[np.var(X[support])]])
location = np.array([location])
# get precision matrix in an optimized way
precision = linalg.pinvh(covariance)
dist = (np.dot(X_centered, precision) * (X_centered)).sum(axis=1)
else:
support = np.ones(n_samples, dtype=bool)
covariance = np.asarray([[np.var(X)]])
location = np.asarray([np.mean(X)])
X_centered = X - location
# get precision matrix in an optimized way
precision = linalg.pinvh(covariance)
dist = (np.dot(X_centered, precision) * (X_centered)).sum(axis=1)
# Starting FastMCD algorithm for p-dimensional case
if (n_samples > 500) and (n_features > 1):
# 1. Find candidate supports on subsets
# a. split the set in subsets of size ~ 300
n_subsets = n_samples // 300
n_samples_subsets = n_samples // n_subsets
samples_shuffle = random_state.permutation(n_samples)
h_subset = int(np.ceil(n_samples_subsets *
(n_support / float(n_samples))))
# b. perform a total of 500 trials
n_trials_tot = 500
# c. select 10 best (location, covariance) for each subset
n_best_sub = 10
n_trials = max(10, n_trials_tot // n_subsets)
n_best_tot = n_subsets * n_best_sub
all_best_locations = np.zeros((n_best_tot, n_features))
try:
all_best_covariances = np.zeros((n_best_tot, n_features,
n_features))
except MemoryError:
# The above is too big. Let's try with something much small
# (and less optimal)
all_best_covariances = np.zeros((n_best_tot, n_features,
n_features))
n_best_tot = 10
n_best_sub = 2
for i in range(n_subsets):
low_bound = i * n_samples_subsets
high_bound = low_bound + n_samples_subsets
current_subset = X[samples_shuffle[low_bound:high_bound]]
best_locations_sub, best_covariances_sub, _, _ = select_candidates(
current_subset, h_subset, n_trials,
select=n_best_sub, n_iter=2,
cov_computation_method=cov_computation_method,
random_state=random_state)
subset_slice = np.arange(i * n_best_sub, (i + 1) * n_best_sub)
all_best_locations[subset_slice] = best_locations_sub
all_best_covariances[subset_slice] = best_covariances_sub
# 2. Pool the candidate supports into a merged set
# (possibly the full dataset)
n_samples_merged = min(1500, n_samples)
h_merged = int(np.ceil(n_samples_merged *
(n_support / float(n_samples))))
if n_samples > 1500:
n_best_merged = 10
else:
n_best_merged = 1
# find the best couples (location, covariance) on the merged set
selection = random_state.permutation(n_samples)[:n_samples_merged]
locations_merged, covariances_merged, supports_merged, d = \
select_candidates(
X[selection], h_merged,
n_trials=(all_best_locations, all_best_covariances),
select=n_best_merged,
cov_computation_method=cov_computation_method,
random_state=random_state)
# 3. Finally get the overall best (locations, covariance) couple
if n_samples < 1500:
# directly get the best couple (location, covariance)
location = locations_merged[0]
covariance = covariances_merged[0]
support = np.zeros(n_samples, dtype=bool)
dist = np.zeros(n_samples)
support[selection] = supports_merged[0]
dist[selection] = d[0]
else:
# select the best couple on the full dataset
locations_full, covariances_full, supports_full, d = \
select_candidates(
X, n_support,
n_trials=(locations_merged, covariances_merged),
select=1,
cov_computation_method=cov_computation_method,
random_state=random_state)
location = locations_full[0]
covariance = covariances_full[0]
support = supports_full[0]
dist = d[0]
elif n_features > 1:
# 1. Find the 10 best couples (location, covariance)
# considering two iterations
n_trials = 30
n_best = 10
locations_best, covariances_best, _, _ = select_candidates(
X, n_support, n_trials=n_trials, select=n_best, n_iter=2,
cov_computation_method=cov_computation_method,
random_state=random_state)
# 2. Select the best couple on the full dataset amongst the 10
locations_full, covariances_full, supports_full, d = select_candidates(
X, n_support, n_trials=(locations_best, covariances_best),
select=1, cov_computation_method=cov_computation_method,
random_state=random_state)
location = locations_full[0]
covariance = covariances_full[0]
support = supports_full[0]
dist = d[0]
return location, covariance, support, dist
class MinCovDet(EmpiricalCovariance):
"""Minimum Covariance Determinant (MCD): robust estimator of covariance.
The Minimum Covariance Determinant covariance estimator is to be applied
on Gaussian-distributed data, but could still be relevant on data
drawn from a unimodal, symmetric distribution. It is not meant to be used
with multi-modal data (the algorithm used to fit a MinCovDet object is
likely to fail in such a case).
One should consider projection pursuit methods to deal with multi-modal
datasets.
Read more in the :ref:`User Guide <robust_covariance>`.
Parameters
----------
store_precision : bool
Specify if the estimated precision is stored.
assume_centered : Boolean
If True, the support of the robust location and the covariance
estimates is computed, and a covariance estimate is recomputed from
it, without centering the data.
Useful to work with data whose mean is significantly equal to
zero but is not exactly zero.
If False, the robust location and covariance are directly computed
with the FastMCD algorithm without additional treatment.
support_fraction : float, 0 < support_fraction < 1
The proportion of points to be included in the support of the raw
MCD estimate. Default is None, which implies that the minimum
value of support_fraction will be used within the algorithm:
[n_sample + n_features + 1] / 2
random_state : int, RandomState instance or None, optional (default=None)
If int, random_state is the seed used by the random number generator;
If RandomState instance, random_state is the random number generator;
If None, the random number generator is the RandomState instance used
by `np.random`.
Attributes
----------
raw_location_ : array-like, shape (n_features,)
The raw robust estimated location before correction and re-weighting.
raw_covariance_ : array-like, shape (n_features, n_features)
The raw robust estimated covariance before correction and re-weighting.
raw_support_ : array-like, shape (n_samples,)
A mask of the observations that have been used to compute
the raw robust estimates of location and shape, before correction
and re-weighting.
location_ : array-like, shape (n_features,)
Estimated robust location
covariance_ : array-like, shape (n_features, n_features)
Estimated robust covariance matrix
precision_ : array-like, shape (n_features, n_features)
Estimated pseudo inverse matrix.
(stored only if store_precision is True)
support_ : array-like, shape (n_samples,)
A mask of the observations that have been used to compute
the robust estimates of location and shape.
dist_ : array-like, shape (n_samples,)
Mahalanobis distances of the training set (on which `fit` is called)
observations.
References
----------
.. [Rouseeuw1984] `P. J. Rousseeuw. Least median of squares regression.
J. Am Stat Ass, 79:871, 1984.`
.. [Rousseeuw] `A Fast Algorithm for the Minimum Covariance Determinant
Estimator, 1999, American Statistical Association and the American
Society for Quality, TECHNOMETRICS`
.. [ButlerDavies] `R. W. Butler, P. L. Davies and M. Jhun,
Asymptotics For The Minimum Covariance Determinant Estimator,
The Annals of Statistics, 1993, Vol. 21, No. 3, 1385-1400`
"""
_nonrobust_covariance = staticmethod(empirical_covariance)
def __init__(self, store_precision=True, assume_centered=False,
support_fraction=None, random_state=None):
self.store_precision = store_precision
self.assume_centered = assume_centered
self.support_fraction = support_fraction
self.random_state = random_state
[docs] def fit(self, X, y=None):
"""Fits a Minimum Covariance Determinant with the FastMCD algorithm.
Parameters
----------
X : array-like, shape = [n_samples, n_features]
Training data, where n_samples is the number of samples
and n_features is the number of features.
y : not used, present for API consistence purpose.
Returns
-------
self : object
Returns self.
"""
X = check_array(X, ensure_min_samples=2, estimator='MinCovDet')
random_state = check_random_state(self.random_state)
n_samples, n_features = X.shape
# check that the empirical covariance is full rank
if (linalg.svdvals(np.dot(X.T, X)) > 1e-8).sum() != n_features:
warnings.warn("The covariance matrix associated to your dataset "
"is not full rank")
# compute and store raw estimates
raw_location, raw_covariance, raw_support, raw_dist = fast_mcd(
X, support_fraction=self.support_fraction,
cov_computation_method=self._nonrobust_covariance,
random_state=random_state)
if self.assume_centered:
raw_location = np.zeros(n_features)
raw_covariance = self._nonrobust_covariance(X[raw_support],
assume_centered=True)
# get precision matrix in an optimized way
precision = linalg.pinvh(raw_covariance)
raw_dist = np.sum(np.dot(X, precision) * X, 1)
self.raw_location_ = raw_location
self.raw_covariance_ = raw_covariance
self.raw_support_ = raw_support
self.location_ = raw_location
self.support_ = raw_support
self.dist_ = raw_dist
# obtain consistency at normal models
self.correct_covariance(X)
# re-weight estimator
self.reweight_covariance(X)
return self
def correct_covariance(self, data):
"""Apply a correction to raw Minimum Covariance Determinant estimates.
Correction using the empirical correction factor suggested
by Rousseeuw and Van Driessen in [RVD]_.
Parameters
----------
data : array-like, shape (n_samples, n_features)
The data matrix, with p features and n samples.
The data set must be the one which was used to compute
the raw estimates.
References
----------
.. [RVD] `A Fast Algorithm for the Minimum Covariance
Determinant Estimator, 1999, American Statistical Association
and the American Society for Quality, TECHNOMETRICS`
Returns
-------
covariance_corrected : array-like, shape (n_features, n_features)
Corrected robust covariance estimate.
"""
correction = np.median(self.dist_) / chi2(data.shape[1]).isf(0.5)
covariance_corrected = self.raw_covariance_ * correction
self.dist_ /= correction
return covariance_corrected
def reweight_covariance(self, data):
"""Re-weight raw Minimum Covariance Determinant estimates.
Re-weight observations using Rousseeuw's method (equivalent to
deleting outlying observations from the data set before
computing location and covariance estimates) described
in [RVDriessen]_.
Parameters
----------
data : array-like, shape (n_samples, n_features)
The data matrix, with p features and n samples.
The data set must be the one which was used to compute
the raw estimates.
References
----------
.. [RVDriessen] `A Fast Algorithm for the Minimum Covariance
Determinant Estimator, 1999, American Statistical Association
and the American Society for Quality, TECHNOMETRICS`
Returns
-------
location_reweighted : array-like, shape (n_features, )
Re-weighted robust location estimate.
covariance_reweighted : array-like, shape (n_features, n_features)
Re-weighted robust covariance estimate.
support_reweighted : array-like, type boolean, shape (n_samples,)
A mask of the observations that have been used to compute
the re-weighted robust location and covariance estimates.
"""
n_samples, n_features = data.shape
mask = self.dist_ < chi2(n_features).isf(0.025)
if self.assume_centered:
location_reweighted = np.zeros(n_features)
else:
location_reweighted = data[mask].mean(0)
covariance_reweighted = self._nonrobust_covariance(
data[mask], assume_centered=self.assume_centered)
support_reweighted = np.zeros(n_samples, dtype=bool)
support_reweighted[mask] = True
self._set_covariance(covariance_reweighted)
self.location_ = location_reweighted
self.support_ = support_reweighted
X_centered = data - self.location_
self.dist_ = np.sum(
np.dot(X_centered, self.get_precision()) * X_centered, 1)
return location_reweighted, covariance_reweighted, support_reweighted
