Toward Optimal Feature Selection
Daphne KollerMehran Sahami
Develops an information-theoretic filter algorithm that efficiently eliminates both irrelevant and redundant features, providing a theoretically grounded solution for high-dimensional classification tasks without the computational burden of wrapper methods.
Modern classification tasks frequently involve datasets with hundreds or thousands of variables, many of which are irrelevant or redundant. High-dimensional data significantly slows model training—often exponentially—and increases the risk of overfitting, especially when available training samples are limited. Existing methods to select useful variables either scale poorly due to computationally prohibitive iterative model training or fail to eliminate redundant, correlated features.
The article develops and evaluates an information-theoretic framework and an efficient heuristic algorithm to eliminate uninformative variables prior to model training without sacrificing predictive accuracy.
The proposed method operates as a preprocessing filter using backward elimination. Rooted in information theory, it measures information loss via cross-entropy (Kullback-Leibler distance) and leverages the concept of a Markov blanket—a subset of features that renders a target feature conditionally independent of the class and all other variables. Because computing exact Markov blankets across all features is computationally intractable, the authors developed a heuristic algorithm that conditions on a small set of strongly correlated features to approximate information loss. The approach was evaluated on synthetic benchmarks (Corral, LED-24), standard benchmark datasets (Congressional Voting, DNA), and two high-dimensional text classification subsets from the Reuters corpus comprising over 1,600 features, comparing downstream classification performance against Naive Bayes and C4.5 decision tree models.
The evaluation yielded several key findings: First, the filter drastically compressed feature spaces while maintaining or improving predictive accuracy, notably reducing the DNA dataset from 180 features down to 30 or 80 features with equal or improved model accuracy. Second, the backward elimination approach successfully handled complex feature interactions where traditional forward selection failed, such as correctly removing misleading correlated features in the Corral dataset to boost C4.5 decision tree accuracy from 81.2% to 100%. Third, in high-dimensional text datasets, the algorithm removed approximately 1,000 features (reducing feature count by roughly 60%) in about 1.5 hours on standard hardware, whereas wrapper-based alternatives would require estimated thousands of hours. Finally, on the DNA benchmark, the algorithm performed feature reduction in 6 to 15 minutes, representing a two-order-of-magnitude runtime reduction compared to a 15-hour wrapper approach that achieved comparable accuracy.
These findings demonstrate that information-theoretic filtering enables organizations to deploy machine learning on high-dimensional domains, such as text classification and bioinformatics, at a fraction of the computational overhead and runtime cost of existing wrapper methods. Because the selection process is decoupled from specific downstream classifiers, the reduced feature subset can be generated once and shared across multiple modeling algorithms without retraining.
Organizations handling massive feature sets should adopt this filtering approach as a standard preprocessing step to prune noisy and redundant inputs before deploying complex or computationally heavy classification algorithms. When maximum accuracy is paramount, teams can use this filter to rapidly discard the vast majority of irrelevant features before applying specialized wrapper searches on the remaining compact subset. Practitioners should also evaluate whether simple, unconditioned feature selection suffices for domains with minimal feature interaction to further save preprocessing time.
The primary limitation of the algorithm is its heuristic approximation of Markov blankets, which uses a fixed number of conditioning variables. Increasing the conditioning set size can fragment small training samples and cause non-monotonic fluctuations in estimated feature relevance. Consequently, users should exercise caution and tune the conditioning parameter appropriately when working with limited training data.
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- Paper: Supervised and Unsupervised Discretization of Continuous Features, James Dougherty et al. (1995). It provides essential background on using entropy-based discretization for machine learning features, a common preprocessing requirement for information-theoretic feature selection methods.
- Paper: Multi-Interval Discretization of Continuous-Valued Attributes for Classification Learning, Usama M. Fayyad et al. (1993). It introduces principled information-theoretic and minimum description length criteria for continuous attribute partitioning, laying algorithmic groundwork for information-based feature processing.
- Paper: Induction of Decision Trees, J. R. Quinlan (1986). It introduces the classic information gain criterion for evaluating attribute relevance from data, which underpins information-theoretic subset selection heuristics.
- Paper: Feature selection based on mutual information criteria of max-dependency, max-relevance, and min-redundancy, Hanchuan Peng et al. (2003). This work formalizes and refines information-theoretic subset selection by introducing the minimal-redundancy-maximal-relevance (mRMR) framework to approximate optimal statistical dependency.
- Paper: Feature Selection for High-Dimensional Data: A Fast Correlation-Based Filter Solution, Lei Yu et al. (2003). It advances information-theoretic subset filtering by formulating the Fast Correlation-Based Filter (FCBF) using symmetrical uncertainty to rapidly eliminate redundant and irrelevant features.
- Paper: Correlation-based Feature Selection for Discrete and Numeric Class Machine Learning, Mark A. Hall (1999). It extends fast subset-evaluation concepts by developing a correlation-based filter heuristic that explicitly penalizes inter-feature redundancy.
- Paper: An Introduction to Variable and Feature Selection, Isabelle M Guyon et al. (2003). It synthesizes subsequent progress across feature ranking and subset selection algorithms into a comprehensive taxonomy and practical methodology.
- Paper: Feature Selection: Evaluation, Application, and Small Sample Performance, Anil K. Jain et al. (1997). It systematically compares and benchmarks sequential search algorithms for feature subset selection across complex synthetic and real-world distributions.
- Paper: Toward integrating feature selection algorithms for classification and clustering, Huan Liu et al. (2005). It builds an integrative categorization framework that unifies subsequent filter, wrapper, and hybrid feature selection methods across tasks.
- Paper: An Extensive Empirical Study of Feature Selection Metrics for Text Classification, George Forman (2003). It conducts a large-scale empirical evaluation of information gain and alternative feature selection criteria on high-dimensional text data.
