Automatic subspace clustering of high dimensional data for data mining applications
Rakesh AgrawalJohannes GehrkeDimitrios GunopulosPrabhakar Raghavan
Introduces CLIQUE, a foundational density-based subspace clustering algorithm that automatically finds clusters embedded in high-dimensional data subspaces and outputs interpretable DNF descriptions with linear scalability.
CLIQUE is a new clustering algorithm developed to meet the specific needs of data mining on high-dimensional datasets, where clusters often exist only in subspaces formed by subsets of attributes rather than in the full space. Traditional partitional and hierarchical methods, along with recent scalable techniques such as BIRCH and DBSCAN, typically examine all dimensions at once; this approach fails when many attributes contain noise or uniform values, because the average point density becomes too low and distance functions lose effectiveness. The work therefore set out to create an automatic procedure that identifies dense regions in the most relevant subspaces, produces concise and interpretable descriptions of those regions, scales to large data volumes, remains insensitive to input order, and makes no assumptions about the underlying data distribution.
The algorithm proceeds in three main stages. It first partitions each dimension into equal-length intervals and uses a bottom-up, level-wise search—modeled on the monotonicity property that any dense unit in k dimensions must project to dense units in every (k–1)-dimensional subspace—to locate candidate dense units without enumerating every possible subspace. An MDL-based pruning step then discards subspaces whose total coverage of data points falls below an automatically chosen threshold, greatly reducing the number of units examined. Connected dense units within each retained subspace are grouped into clusters via depth-first search, and each cluster is covered by the smallest number of maximal axis-parallel rectangles whose union yields a compact DNF expression.
Experiments on synthetic data with up to 500 000 records and 100 dimensions demonstrated linear scaling with database size and acceptable growth with dimensionality once pruning is applied; the method recovered all embedded clusters while BIRCH and DBSCAN missed most of them once the ambient dimensionality exceeded roughly ten. On four real datasets from insurance, retail, and banking domains, CLIQUE consistently surfaced meaningful clusters lying in subspaces whose dimensionality was far lower than that of the original tables. These results indicate that automatic subspace clustering can be performed at practical cost and with higher accuracy than full-space methods when the data contain the mixture of relevant and irrelevant attributes typical of modern analytical repositories.
The principal limitations are that running time remains exponential in the highest dimensionality of any dense unit and that aggressive MDL pruning can, in principle, discard a subspace containing a cluster. Parameter selection for grid granularity and density threshold still requires user judgment, although the authors note that modest ranges usually suffice. Overall is high for the reported synthetic and real-data regimes, but practitioners should verify results on new domains with a modest pilot before relying on the output for downstream decisions.
Next steps supported by the paper include developing quantitative criteria for ranking subspaces by cluster quality, adding system-level assistance for parameter choice, and exploring maximal-itemset techniques to locate only the highest-dimensional dense units without enumerating all their projections. These extensions would further lower the barrier to routine use of subspace clustering in operational data-mining pipelines.
- Paper: BIRCH: an efficient data clustering method for very large databases, Tian Zhang et al. (1996). BIRCH provides foundational techniques for data partitioning and memory-efficient cluster summaries that directly preceded scalable subspace methods like CLIQUE.
- Paper: Mining association rules between sets of items in large databases, R. Agrawal et al. (1993). Association rule mining introduces the underlying frequent-itemset discovery principles that CLIQUE adapts to identify dense clusters in high-dimensional subspaces.
- Paper: Robust Recovery of Subspace Structures by Low-Rank Representation, Guangcan Liu et al. (2010). Low-rank representation extends subspace analysis beyond grid-based methods by using convex optimization to recover clean linear subspaces from corrupted high-dimensional data.
- Paper: Unsupervised Deep Embedding for Clustering Analysis, Junyuan Xie et al. (2015). Deep embedded clustering advances beyond classical subspace and partitioning algorithms by jointly learning non-linear feature representations and cluster assignments via neural networks.
