## Abstract

The amount of training-data is one of the key factors which determines the generalization capacity of learning algorithms. Intuitively, one expects the error rate to decrease as the amount of training-data increases. Perhaps surprisingly, natural attempts to formalize this intuition give rise to interesting and challenging mathematical questions. For example, in their classical book on pattern recognition, Devroye, Györfi, and Lugosi (1996) ask whether there exists a monotone Bayes-consistent algorithm. This question remained open for over 25 years, until recently Pestov (2021) resolved it for binary classification, using an intricate construction of a monotone Bayes-consistent algorithm. We derive a general result in multiclass classification, showing that every learning algorithm A can be transformed to a monotone one with similar performance. Further, the transformation is efficient and only uses a black-box oracle access to A. This demonstrates that one can provably avoid non-monotonic behaviour without compromising performance, thus answering questions asked by Devroye, Györfi, and Lugosi (1996), Viering, Mey, and Loog (2019), Viering and Loog (2021), and by Mhammedi (2021). Our general transformation readily implies monotone learners in a variety of contexts: for example, Pestov’s result follows by applying it on any Bayes-consistent algorithm (e.g., k-NearestNeighbours). In fact, our transformation extends Pestov’s result to classification tasks with an arbitrary number of labels. This is in contrast with Pestov’s work which is tailored to binary classification. In addition, we provide uniform bounds on the error of the monotone algorithm. This makes our transformation applicable in distribution-free settings. For example, in PAC learning it implies that every learnable class admits a monotone PAC learner. This resolves questions asked by Viering, Mey, and Loog (2019); Viering and Loog (2021); Mhammedi (2021).

Original language | English |
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Pages (from-to) | 842-866 |

Number of pages | 25 |

Journal | Proceedings of Machine Learning Research |

Volume | 178 |

State | Published - 2022 |

Event | 35th Conference on Learning Theory, COLT 2022 - London, United Kingdom Duration: 2 Jul 2022 → 5 Jul 2022 |

### Funding

Funders | Funder number |
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European Union’s Horizon 2022 research and innovation program | 101041711, 2258/19 |

Israel PBC-VATAT | |

MLIS | 1595-19 |

Technion Center for Machine Learning and Intelligent Systems | |

Yandex Initiative for Machine Learning | 1225/20 |

United States - Israel Binational Science Foundation | |

Horizon 2020 Framework Programme | 882396 |

Horizon 2020 Framework Programme | |

Blavatnik Family Foundation | 1871/19 |

Blavatnik Family Foundation | |

European Commission | |

United States-Israel Binational Science Foundation | |

Israel Science Foundation | 993/17 |

Israel Science Foundation | |

Tel Aviv University |

## Keywords

- Bayes consistency
- Learning curve
- Monotonicity
- PAC learning