Abstract
Efficient and reliable methods for distinguishing outliers in data remain crucial for data analysis. Although supervised methods based on neural networks have gained recent traction, unsupervised methods such as the kNN outlier method and local outlier factor (LOF) remain state-of-the-art solutions according to different standardized benchmarks. Unfortunately, exact outlier detection through nearest neighbor search queries provides a scalability bottleneck for the high-dimensional, big datasets that are routinely analyzed in data science applications. This paper explores benefits and limitations of using approximate nearest neighbor search via Hierarchical Navigable Small World graphs (HNSW) to overcome this scalability barrier. We evaluate direct implementations that compute the kNN and LOF score from approximate neighborhoods and show the robustness of the outlier detection even in settings where the approximation is far away from the exact neighborhoods. Furthermore, we design white-box methods that compute the outlier scores directly from the underlying graph. These methods show much more variability in the quality of the outlier scores and open new ground for the development of task-aware tools based on approximate nearest neighbor search techniques.
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Building the HNSW index using hnswlib in a multicore environment lead to slightly different graphs due to different pruning orders. This non-determinism resulted in different quality scores for the same hyperparameter settings.
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Acknowledgments
The initial research question was investigated in a Master thesis by Davidsson et al. [1]. We thank the anonymous reviewers for the careful comments on the paper. This project received funding from the Innovation Fund Denmark for the project DIREC (9142-00001B).
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Okkels, C.B., Aumüller, M., Zimek, A. (2025). On the Design of Scalable Outlier Detection Methods Using Approximate Nearest Neighbor Graphs. In: Chávez, E., Kimia, B., Lokoč, J., Patella, M., Sedmidubsky, J. (eds) Similarity Search and Applications. SISAP 2024. Lecture Notes in Computer Science, vol 15268. Springer, Cham. https://doi.org/10.1007/978-3-031-75823-2_14
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