Here, we will learn about what is anomaly detection in Sklearn and how it is used in identification of the data points.
Anomaly detection is a technique used to identify data points in dataset that does not fit well with the rest of the data. It has many applications in business such as fraud detection, intrusion detection, system health monitoring, surveillance, and predictive maintenance. Anomalies, which are also called outlier, can be divided into following three categories −
Point anomalies − It occurs when an individual data instance is considered as anomalous w.r.t the rest of the data.
Contextual anomalies − Such kind of anomaly is context specific. It occurs if a data instance is anomalous in a specific context.
Collective anomalies − It occurs when a collection of related data instances is anomalous w.r.t entire dataset rather than individual values.
Two methods namely outlier detection and novelty detection can be used for anomaly detection. It’s necessary to see the distinction between them.
The training data contains outliers that are far from the rest of the data. Such outliers are defined as observations. That’s the reason, outlier detection estimators always try to fit the region having most concentrated training data while ignoring the deviant observations. It is also known as unsupervised anomaly detection.
It is concerned with detecting an unobserved pattern in new observations which is not included in training data. Here, the training data is not polluted by the outliers. It is also known as semisupervised anomaly detection.
There are set of ML tools, provided by scikitlearn, which can be used for both outlier detection as well novelty detection. These tools first implementing object learning from the data in an unsupervised by using fit () method as follows −
estimator.fit(X_train)
Now, the new observations would be sorted as inliers (labeled 1) or outliers (labeled 1) by using predict() method as follows −
estimator.fit(X_test)
The estimator will first compute the raw scoring function and then predict method will make use of threshold on that raw scoring function. We can access this raw scoring function with the help of score_sample method and can control the threshold by contamination parameter.
We can also define decision_function method that defines outliers as negative value and inliers as nonnegative value.
estimator.decision_function(X_test)
Let us begin by understanding what an elliptic envelop is.
This algorithm assume that regular data comes from a known distribution such as Gaussian distribution. For outlier detection, Scikitlearn provides an object named covariance.EllipticEnvelop.
This object fits a robust covariance estimate to the data, and thus, fits an ellipse to the central data points. It ignores the points outside the central mode.
Following table consist the parameters used by sklearn. covariance.EllipticEnvelop method −
Sr.No  Parameter & Description 

1 
store_precision − Boolean, optional, default = True We can specify it if the estimated precision is stored. 
2 
assume_centered − Boolean, optional, default = False If we set it False, it will compute the robust location and covariance directly with the help of FastMCD algorithm. On the other hand, if set True, it will compute the support of robust location and covarian. 
3 
support_fraction − float in (0., 1.), optional, default = None This parameter tells the method that how much proportion of points to be included in the support of the raw MCD estimates. 
4 
contamination − float in (0., 1.), optional, default = 0.1 It provides the proportion of the outliers in the data set. 
5 
random_state − int, RandomState instance or None, optional, default = none This parameter represents the seed of the pseudo random number generated which is used while shuffling the data. Followings are the options −

Following table consist the attributes used by sklearn. covariance.EllipticEnvelop method −
Sr.No  Attributes & Description 

1 
support_ − arraylike, shape(n_samples,) It represents the mask of the observations used to compute robust estimates of location and shape. 
2 
location_ − arraylike, shape (n_features) It returns the estimated robust location. 
3 
covariance_ − arraylike, shape (n_features, n_features) It returns the estimated robust covariance matrix. 
4 
precision_ − arraylike, shape (n_features, n_features) It returns the estimated pseudo inverse matrix. 
5 
offset_ − float It is used to define the decision function from the raw scores. decision_function = score_samples offset_ 
Implementation Example
import numpy as np^M from sklearn.covariance import EllipticEnvelope^M true_cov = np.array([[.5, .6],[.6, .4]]) X = np.random.RandomState(0).multivariate_normal(mean = [0, 0], cov=true_cov,size=500) cov = EllipticEnvelope(random_state = 0).fit(X)^M # Now we can use predict method. It will return 1 for an inlier and 1 for an outlier. cov.predict([[0, 0],[2, 2]])
Output
array([ 1, 1])
In case of highdimensional dataset, one efficient way for outlier detection is to use random forests. The scikitlearn provides ensemble.IsolationForest method that isolates the observations by randomly selecting a feature. Afterwards, it randomly selects a value between the maximum and minimum values of the selected features.
Here, the number of splitting needed to isolate a sample is equivalent to path length from the root node to the terminating node.
Followings table consist the parameters used by sklearn. ensemble.IsolationForest method −
Sr.No  Parameter & Description 

1 
n_estimators − int, optional, default = 100 It represents the number of base estimators in the ensemble. 
2 
max_samples − int or float, optional, default = “auto” It represents the number of samples to be drawn from X to train each base estimator. If we choose int as its value, it will draw max_samples samples. If we choose float as its value, it will draw max_samples ∗ 𝑋.shape[0] samples. And, if we choose auto as its value, it will draw max_samples = min(256,n_samples). 
3 
support_fraction − float in (0., 1.), optional, default = None This parameter tells the method that how much proportion of points to be included in the support of the raw MCD estimates. 
4 
contamination − auto or float, optional, default = auto It provides the proportion of the outliers in the data set. If we set it default i.e. auto, it will determine the threshold as in the original paper. If set to float, the range of contamination will be in the range of [0,0.5]. 
5 
random_state − int, RandomState instance or None, optional, default = none This parameter represents the seed of the pseudo random number generated which is used while shuffling the data. Followings are the options −

6 
max_features − int or float, optional (default = 1.0) It represents the number of features to be drawn from X to train each base estimator. If we choose int as its value, it will draw max_features features. If we choose float as its value, it will draw max_features * X.shape[𝟏] samples. 
7  bootstrap − Boolean, optional (default = False) Its default option is False which means the sampling would be performed without replacement. And on the other hand, if set to True, means individual trees are fit on a random subset of the training data sampled with replacement. 
8 
n_jobs − int or None, optional (default = None) It represents the number of jobs to be run in parallel for fit() and predict() methods both. 
9 
verbose − int, optional (default = 0) This parameter controls the verbosity of the tree building process. 
10 
warm_start − Bool, optional (default=False) If warm_start = true, we can reuse previous calls solution to fit and can add more estimators to the ensemble. But if is set to false, we need to fit a whole new forest. 
Following table consist the attributes used by sklearn. ensemble.IsolationForest method −
Sr.No  Attributes & Description 

1 
estimators_ − list of DecisionTreeClassifier Providing the collection of all fitted subestimators. 
2 
max_samples_ − integer It provides the actual number of samples used. 
3 
offset_ − float It is used to define the decision function from the raw scores. decision_function = score_samples offset_ 
Implementation Example
The Python script below will use sklearn. ensemble.IsolationForest method to fit 10 trees on given data
from sklearn.ensemble import IsolationForest import numpy as np X = np.array([[1, 2], [3, 3], [3, 4], [0, 0], [50, 60]]) OUTDClf = IsolationForest(n_estimators = 10) OUTDclf.fit(X)
Output
IsolationForest( behaviour = 'old', bootstrap = False, contamination='legacy', max_features = 1.0, max_samples = 'auto', n_estimators = 10, n_jobs=None, random_state = None, verbose = 0 )
Local Outlier Factor (LOF) algorithm is another efficient algorithm to perform outlier detection on high dimension data. The scikitlearn provides neighbors.LocalOutlierFactor method that computes a score, called local outlier factor, reflecting the degree of anomality of the observations. The main logic of this algorithm is to detect the samples that have a substantially lower density than its neighbors. Thats why it measures the local density deviation of given data points w.r.t. their neighbors.
Followings table consist the parameters used by sklearn. neighbors.LocalOutlierFactor method
Sr.No  Parameter & Description 

1 
n_neighbors − int, optional, default = 20 It represents the number of neighbors use by default for kneighbors query. All samples would be used if . 
2 
algorithm − optional Which algorithm to be used for computing nearest neighbors.

3 
leaf_size − int, optional, default = 30 The value of this parameter can affect the speed of the construction and query. It also affects the memory required to store the tree. This parameter is passed to BallTree or KdTree algorithms. 
4 
contamination − auto or float, optional, default = auto It provides the proportion of the outliers in the data set. If we set it default i.e. auto, it will determine the threshold as in the original paper. If set to float, the range of contamination will be in the range of [0,0.5]. 
5 
metric − string or callable, default It represents the metric used for distance computation. 
6 
P − int, optional (default = 2) It is the parameter for the Minkowski metric. P=1 is equivalent to using manhattan_distance i.e. L1, whereas P=2 is equivalent to using euclidean_distance i.e. L2. 
7 
novelty − Boolean, (default = False) By default, LOF algorithm is used for outlier detection but it can be used for novelty detection if we set novelty = true. 
8 
n_jobs − int or None, optional (default = None) It represents the number of jobs to be run in parallel for fit() and predict() methods both. 
Following table consist the attributes used by sklearn.neighbors.LocalOutlierFactor method −
Sr.No  Attributes & Description 

1 
negative_outlier_factor_ − numpy array, shape(n_samples,) Providing opposite LOF of the training samples. 
2 
n_neighbors_ − integer It provides the actual number of neighbors used for neighbors queries. 
3 
offset_ − float It is used to define the binary labels from the raw scores. 
Implementation Example
The Python script given below will use sklearn.neighbors.LocalOutlierFactor method to construct NeighborsClassifier class from any array corresponding our data set
from sklearn.neighbors import NearestNeighbors samples = [[0., 0., 0.], [0., .5, 0.], [1., 1., .5]] LOFneigh = NearestNeighbors(n_neighbors = 1, algorithm = "ball_tree",p=1) LOFneigh.fit(samples)
Output
NearestNeighbors( algorithm = 'ball_tree', leaf_size = 30, metric='minkowski', metric_params = None, n_jobs = None, n_neighbors = 1, p = 1, radius = 1.0 )
Example
Now, we can ask from this constructed classifier is the closet point to [0.5, 1., 1.5] by using the following python script −
print(neigh.kneighbors([[.5, 1., 1.5]])
Output
(array([[1.7]]), array([[1]], dtype = int64))
The OneClass SVM, introduced by Schölkopf et al., is the unsupervised Outlier Detection. It is also very efficient in highdimensional data and estimates the support of a highdimensional distribution. It is implemented in the Support Vector Machines module in the Sklearn.svm.OneClassSVM object. For defining a frontier, it requires a kernel (mostly used is RBF) and a scalar parameter.
For better understanding let's fit our data with svm.OneClassSVM object −
from sklearn.svm import OneClassSVM X = [[0], [0.89], [0.90], [0.91], [1]] OSVMclf = OneClassSVM(gamma = 'scale').fit(X)
Now, we can get the score_samples for input data as follows −
OSVMclf.score_samples(X)
array([1.12218594, 1.58645126, 1.58673086, 1.58645127, 1.55713767])