Lära Challenges in Anomaly Detection | Foundations of Outlier and Novelty Detection

Anomaly detection faces three main challenges:

Class imbalance: Anomalies are extremely rare, so models mostly see normal data and may fail to recognize outliers;
Contamination: The "normal" class often contains hidden anomalies, which confuses models and reduces detection accuracy;
Scarcity of labeled anomalies: Few labeled examples make supervised training and evaluation difficult.

These factors limit standard machine learning approaches and require special care in designing and evaluating anomaly detection systems.

Note

Mitigation strategies for anomaly detection challenges:

Use unsupervised learning algorithms that do not require labeled anomalies;
Apply robust evaluation metrics such as precision, recall, and ROC-AUC that account for class imbalance;
Employ data cleaning and preprocessing steps to minimize contamination in training data;
Consider semi-supervised approaches when a small set of labeled anomalies is available;
Use domain knowledge to guide feature selection and post-processing.


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import numpy as np
import pandas as pd
from sklearn.datasets import make_classification

# Create a synthetic dataset with strong class imbalance and contamination
X, y = make_classification(n_samples=1000, n_features=2, 
                           n_informative=2, n_redundant=0,
                           n_clusters_per_class=1, weights=[0.98, 0.02],
                           flip_y=0, random_state=42)

# Introduce contamination: flip a small fraction of normal labels to anomaly
contamination_rate = 0.01  # 1% contamination
n_contaminated = int(contamination_rate * sum(y == 0))
contaminated_idx = np.random.choice(np.where(y == 0)[0], n_contaminated, replace=False)
y[contaminated_idx] = 1  # contaminate normal data with anomalies

# Count class distribution after contamination
unique, counts = np.unique(y, return_counts=True)
class_distribution = dict(zip(unique, counts))

print("Class distribution after contamination:", class_distribution)
print("Contamination rate (actual): {:.2f}%".format(
    100 * counts[1] / (counts[0] + counts[1])))

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