## On Tail Decay Rate Estimation of Loss Function Distributions

** Etrit Haxholli, Marco Lorenzi**; 25(25):1−47, 2024.

### Abstract

The study of loss-function distributions is critical to characterize a model's behaviour on a given machine-learning problem. While model quality is commonly measured by the average loss assessed on a testing set, this quantity does not ascertain the existence of the mean of the loss distribution. Conversely, the existence of a distribution's statistical moments can be verified by examining the thickness of its tails. Cross-validation schemes determine a family of testing loss distributions conditioned on the training sets. By marginalizing across training sets, we can recover the overall (marginal) loss distribution, whose tail-shape we aim to estimate. Small sample-sizes diminish the reliability and efficiency of classical tail-estimation methods like Peaks-Over-Threshold, and we demonstrate that this effect is notably significant when estimating tails of marginal distributions composed of conditional distributions with substantial tail-location variability. We mitigate this problem by utilizing a result we prove: under certain conditions, the marginal-distribution's tail-shape parameter is the maximum tail-shape parameter across the conditional distributions underlying the marginal. We label the resulting approach as `cross-tail estimation (CTE)'. We test CTE in a series of experiments on simulated and real data showing the improved robustness and quality of tail estimation as compared to classical approaches.

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