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Confidence_AD

Confidence_AD (Confidence in Anomaly Detection) is a GitHub repository containing the ExCeeD [1] algorithm. It refers to the paper titled Quantifying the confidence of anomaly detectors in their example-wise predictions.

Read the pdf here: [pdf].

Abstract

Anomaly detection focuses on identifying examples in the data that somehow deviate from what is expected or typical. Algorithms for this task usually assign a score to each example that represents how anomalous the example is. Then, a threshold on the scores turns them into concrete predictions. However, each algorithm uses a different approach to assign the scores, which makes them difficult to interpret and can quickly erode a user's trust in the predictions. Here we introduce ExCeeD [1], an approach for assessing the reliability of any anomaly detector's example-wise predictions in two steps:

  1. it transforms anomaly scores into outlier probabilities by using a Bayesian approach, and
  2. it derives a confidence score for each exemple-wise prediction, which captures the anomaly detector's uncertainty in that prediction.

Contents and usage

The repository contains:

  • ExCeeD.py, a function that allows to get the confidence values;
  • Notebook.ipynb, a notebook showing how to use ExCeeD on an artificial dataset;
  • evaluate_ExCeeD.py, a function used to get the experimental results on benchmark datasets;
  • Experiments.ipynb, a notebook showing how to run the experiments.

To use ExCeeD, import the github repository or simply download the files. You can also find the benchmark datasets inside the folder Benchmark_Datasets or at this [link].

EXample-wise ConfidEncE of anomaly Detectors (ExCeeD)

Given a dataset with attributes X, an unsupervised anomaly detector assigns to each example an anomaly score, representing its degree of anomalousness. In order to compute the confidence scores, ExCeeD [1] takes into account the anomaly scores. For this reason, the algorithm works with any anomaly detector, assuming that the anomaly scores follow the rule that the higher is the score, the more anomalous is the example. It can also be used in a semi-supervised setting, where examples are partially labeled and even in a supervised setting, where all the examples are labeled.

Given a training dataset X_train (with labels y or not), and a test dataset X_test, the algorithm is applied as follows:

from pyod.models.knn import KNN
from ExCeeD import *

# Estimate the contamination factor (which has to be given), for instance with
contamination = sum(y)/len(y)

# Train an anomaly detector (for instance, here we use kNNO)
detector = KNN().fit(X_train)

# Compute the anomaly scores in the training set
train_scores_knno = detector.decision_function(X_train)

# Compute the anomaly scores in the test set
test_scores_knno = detector.decision_function(X_test)

# Predict the class of each test example
prediction_knno = detector.predict(X_test)

# Estimate the confidence in class predictions with ExCeeD
knno_confidence = ExCeeD(train_scores_knno, test_scores_knno, prediction_knno, contamination)

Dependencies

The ExCeeD function requires the following python packages to be used:

Contact

Contact the author of the paper: lorenzo.perini@kuleuven.be.

References

[1] Perini, L., Vercruyssen, V., Davis, J.: Quantifying the confidence of anomaly detectors in their example-wise predictions. In: The European Conference on Machine Learning and Principles and Practice of Knowledge Discovery in Databases. Springer Verlag (2020).