How Does Pseudo-Labeling Affect the Generalization Error of the Semi-Supervised Gibbs Algorithm?
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This paper provides an exact characterization of the expected generalization error (gen-error) for semi-supervised learning (SSL) with pseudo-labeling via the Gibbs algorithm. This characterization is expressed in terms of the symmetrized KL information between the output hypothesis, the pseudo-labeled dataset, and the labeled dataset. It can be applied to obtain distribution-free upper and lower bounds on the gen-error. Our findings offer new insights that the generalization performance of SSL with pseudo-labeling is affected not only by the information between the output hypothesis and input training data but also by the information shared between the labeled and pseudo-labeled data samples. To deepen our understanding, we further explore two examples – mean estimation and logistic regression. In particular, we analyze how the ratio of the number of unlabeled to labeled data λ affects the gen-error under both scenarios. As λ increases, the gen-error for mean estimation decreases and then saturates at a value larger than when all the samples are labeled, and the gap can be quantified exactly with our analysis, and is dependent on the cross-covariance between the labeled and pseudo-labeled data sample. In logistic regression, the gen-error and the variance component of the excess risk also decrease as λ increases.
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