Computer Sciences Dept.

Adaptively Finding and Combining First-Order Rules for Large, Skewed Data Sets

Louis Oliphant

Inductive Logic Programming (ILP) is a machine-learning approach that uses first-order logic to create human-readable rules from a database of information and a set of positive and negative examples. When working with highly skewed data sets where the negatives vastly outnumber the positives, common metrics such as predictive accuracy and area under the receiver-operator characteristic curves (AUROC) do not work well because these metrics count negatives and positives equally, causing performance on the negative examples to dominate. This thesis explores creating ensembles of rules to maximize area under the recall-precision curves (AURPC), a much better metric that focuses specifically on the coverage and accuracy of labeling the positive examples.

I create an ensemble of rules from a wide range of recall values and combine them to maximize AURPC. My Gleaner algorithm retains a set of rules for each positive seed example where standard ILP methods keep only a single rule. Gleaning rules from those rules that would normally be discarded and combining them into a single ensemble shows improved predictive performance while reducing the number of rules evaluated.

I evaluate several modified search methods for finding sets of clauses that work well together. One method applies a probability distribution over the space of rules and stochastically selects rules more likely to improve Gleanerís predictive performance. A second method follows a boosting framework and weights examples in order to maximize AURPC. Tying together the method of combining rules with the search for good candidate rules shows improvement over the standard Gleaner algorithm.

I apply these first-order ensemble techniques to several data sets from two very different domains. The first data sets come from the Information-Extraction (IE) domain where the task is to find specific relationships in text. The next data sets come from the computer-assisted medical-diagnosis domain. The task is to identify findings on a mammogram as malignant or benign given descriptors of the findings, patient risk factors, radiologistís score, and information from any previous mammograms.

I also include my work with Davis et al.ís SAYU algorithm. I demonstrate methods to improve predictive performance and to increase understanding of malignancy indicators. Inclusion of additional background knowledge that allows for rules to contain ranges of values provides for more complex models that improve predictive performance. I also show that transferred models are able to outperform radiologists at new institutions even when no additional data are available from the new institution. Finally, first-order rules and probability help in improving understanding of malignant indicators. I use these techniques to confirm the importance of high mass density in identifying malignant findings. I also identify surprising pairs of features that perform better than expected at identifying malignant findings than would be expected by looking at the features individually.

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