Which of the following statements best describes how the Cluster-Based Local Outlier Factor (CBLOF) method detects anomalies?

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To analyze how CBLOF detects anomalies, we must compare each statement to the core idea of the method.
Option 1: 'It detects anomalies by evaluating whether points lie in regions of significantly lower density than their neighbors.' While CBLOF relates to clusters and distances, this description aligns more closely with density-based methods like LOF rather than CBLOF’s cluster-centric approach. It emphasizes local density regions rather than cluster structure and size.
Option 2: 'It assigns outlier scores based on the size of the cluster a point belongs to and its distance to the nearest large cluster if it's in a small one.' This matches the CBLOF framework: points in smaller clusters receive higher anomaly scores based on the cluster size, and the distance to larger clusters further modulates the score, capturing how atypical a point’s placement is relative to the global cluster structure.
Option 3: 'It labels points as anomalies if their distance to their k-nearest neighbors is above a threshold.' This describes a typical k-NN-based anomaly detector, which relies on local neighbor distances, not on cluster sizes and distances to large clusters as CBLOF does.
Option 4: 'It flags points as anomalies if they fall outside the convex hull formed by the majority of data points.' This reflects a convex-hull or boundary-based approach, which CBLOF does not specifically use; CBLOF focuses on cluster-driven scoring rather than convex-hull containment.
In summary, option 2 accurately captures CBLOF’s mechanism, while the others describe distinct anomaly detection paradigms that CBLOF does not primarily rely on.

COMP90049_2025_SM2 Lecture 22 Anomaly Detection Practice Quiz

Which of the following statements best describes how the Cluster-Based Local Outlier Factor (CBLOF) method detects anomalies?

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