Graduate Thesis Or Dissertation
 

Understanding and improving error-correcting output coding

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/wh246z090

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  • Error-correcting output coding (ECOC) is a method for converting a k-classsupervised learning problem into a large number L of two-class supervised learningproblems and then combining the results of these L evaluations. Previous researchhas shown that ECOC can dramatically improve the classi cation accuracy of supervisedlearning algorithms that learn to classify data points into one of k 2 classes.An investigation of why the ECOC technique works, particularly when employedwith decision tree learning algorithms, is presented.It is shown that the ECOC method is a compact form of voting amongmultiple hypotheses. The success of the voting depends on that the errors committedby each of the L learned binary functions are substantially uncorrelated.By employing the statistical notions of bias and variance, the generalizationerrors of ECOC are decomposed into bias and variance errors. Like any votingmethod, ECOC reduces variance errors. However, unlike homogeneous voting, whichsimply combines multiple runs of the same learning algorithm, ECOC can alsoreduce bias errors. It is shown that the bias errors in the individual functions are uncorrelated and that this results from non-local behavior of the learning algorithmin splitting the feature space.ECOC is also extended to provide class probability information. The problemof computing these class probabilities can be formulated as an over-constrained systemof linear equations. Least squares methods are applied to solve these equations.Accuracy of the posterior probabilities is demonstrated with overlapping classes anda simple reject option task.
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