CountSketches, Feature Hashing and the Median of Three

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In this paper, we revisit the classic CountSketch method, which is a sparse, random projection that transforms a (high-dimensional) Euclidean vector v to a vector of dimension (2t - 1)s, where t, s > 0 are integer parameters. It is known that a CountSketch allows estimating coordinates of v with variance bounded by parallel to v parallel to/s. For t > 1, the estimator takes the median of 2t - 1 independent estimates, and the probability that the estimate is off by more than 2 parallel to v parallel to/root s is exponentially small in t. This suggests choosing t to be logarithmic in a desired inverse failure probability. However, implementations of CountSketch often use a small, constant t. Previous work only predicts a constant factor improvement in this setting. Our main contribution is a new analysis of CountSketch, showing an improvement in variance to O(min{parallel to v parallel to, parallel to v parallel to/s}) when t > 1. That is, the variance decreases proportionally to s, asymptotically for large enough s.

Original languageEnglish
Title of host publicationProceedings of the 38 th International Conference on Machine Learning
EditorsM Meila, T Zhang
PublisherPMLR
Publication date2021
Pages6011-6020
Publication statusPublished - 2021
Event38th International Conference on Machine Learning (ICML) - Virtual
Duration: 18 Jul 202124 Jul 2021

Conference

Conference38th International Conference on Machine Learning (ICML)
ByVirtual
Periode18/07/202124/07/2021
SeriesProceedings of Machine Learning Research
Volume139
ISSN2640-3498

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