The Pessimistic Limits and Possibilities of Margin-based Losses in Semi-supervised Learning

Part of Advances in Neural Information Processing Systems 31 (NeurIPS 2018)

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Jesse Krijthe, Marco Loog


Consider a classification problem where we have both labeled and unlabeled data available. We show that for linear classifiers defined by convex margin-based surrogate losses that are decreasing, it is impossible to construct \emph{any} semi-supervised approach that is able to guarantee an improvement over the supervised classifier measured by this surrogate loss on the labeled and unlabeled data. For convex margin-based loss functions that also increase, we demonstrate safe improvements \emph{are} possible.