--Performance gap across classes remains a persistent challenge in machine learning, often attributed to variations in class hardness. One way to quantify class hardness is through sample complexity - the minimum number of samples required to effectively learn a given class. Sample complexity theory suggests that class hardness is driven by differences in the amount of data required for generalization. That is, harder classes need substantially more samples to achieve generalization. Therefore, hardness-based resampling is a promising approach to mitigate these performance disparities. While resampling has been studied extensively in data-imbalanced settings, its impact on balanced datasets remains unexplored. This raises the fundamental question whether resampling is effective because it addresses data imbalance or hardness imbalance. We begin addressing this question by introducing class imbalance into balanced datasets and evaluate its effect on performance disparities. We oversample hard classes and undersample easy classes to bring hard classes closer to their sample complexity requirements while maintaining a constant dataset size for fairness. We estimate class-level hardness using the Area Under the Margin (AUM) hardness estimator and leverage it to compute resampling ratios. Using these ratios, we perform hardness-based resampling on the well-known CIF AR-10 and CIF AR-100 datasets. Contrary to theoretical expectations, our results show that hardness-based resampling does not meaningfully affect class-wise performance disparities. T o explain this discrepancy, we conduct detailed analyses to identify key challenges unique to hardness-based imbalance, distinguishing it from traditional data-based imbalance. Our insights help explain why theoretical sample complexity expectations fail to translate into practical performance gains and we provide guidelines for future research. CCESS to large datasets has fueled recent machine learning breakthroughs [1], yet data efficiency remains a critical challenge [2]. Addressing this challenge requires a deep understanding of instance, class, and dataset level hardness. Empirical works reveal large performance gaps across classes [3]-[5] and clear distinctions between easy (e.g. ImageNet) [6], [7] as shown in see Figure 1. Pawel Pukowski is with the University of Sheffield. V enet Osmani is with the Digital Environment Research Institute, Queen Mary University of London. This work has been submitted to the IEEE for possible publication. Figure 1: Training an ensemble of ten ResNet18 networks on CIFAR-10 (left) and CIFAR-100 (right) reveals large recall gaps across classes, despite the balanced nature of these datasets. Paired with significantly larger recall gaps across classes for CIFAR-100 than CIFAR-10, this shows class-and dataset-level hardness discrepancies, which we call hardness-based imbalance.

Deep neural networks are susceptible to adversarial attacks and common corruptions, which undermine their robustness. In order to enhance model resilience against such challenges, Adversarial Training (AT) has emerged as a prominent solution. Nevertheless, adversarial robustness is often attained at the expense of model fairness during AT, i.e., disparity in class-wise robustness of the model. While distinctive classes become more robust towards such adversaries, hard to detect classes suffer. Recently, research has focused on improving model fairness specifically for perturbed images, overlooking the accuracy of the most likely non-perturbed data. Additionally, despite their robustness against the adversaries encountered during model training, state-of-the-art adversarial trained models have difficulty maintaining robustness and fairness when confronted with diverse adversarial threats or common corruptions. In this work, we address the above concerns by introducing a novel approach called Fair Targeted Adversarial Training (FAIR-TAT). We show that using targeted adversarial attacks for adversarial training (instead of untargeted attacks) can allow for more favorable trade-offs with respect to adversarial fairness. Empirical results validate the efficacy of our approach.

The disparity in accuracy between classes in standard training is amplified during adversarial training, a phenomenon termed the robust fairness problem. Existing methodologies aimed to enhance robust fairness by sacrificing the model's performance on easier classes in order to improve its performance on harder ones. However, we observe that under adversarial attacks, the majority of the model's predictions for samples from the worst class are biased towards classes similar to the worst class, rather than towards the easy classes. Through theoretical and empirical analysis, we demonstrate that robust fairness deteriorates as the distance between classes decreases. Motivated by these insights, we introduce the Distance-Aware Fair Adversarial training (DAFA) methodology, which addresses robust fairness by taking into account the similarities between classes. Specifically, our method assigns distinct loss weights and adversarial margins to each class and adjusts them to encourage a trade-off in robustness among similar classes. Experimental results across various datasets demonstrate that our method not only maintains average robust accuracy but also significantly improves the worst robust accuracy, indicating a marked improvement in robust fairness compared to existing methods. Recent studies have revealed the issue of accuracy imbalance among classes (He & Garcia, 2009). This imbalance becomes even more pronounced during adversarial training, which utilizes adversarial examples (Szegedy et al., 2013) to enhance the robustness of the model (Madry et al., 2017). This phenomenon is commonly referred to as "robust fairness problem" (Xu et al., 2021). Existing research has introduced methods inspired by long-tailed (LT) classification studies (He & Garcia, 2009; Zhang et al., 2023) to mitigate the challenge of achieving robust fairness. LT classification tasks tackle the problem of accuracy imbalance among classes, stemming from classifier bias toward classes with a substantial number of samples (head classes) within the LT dataset. The methods proposed for LT classification mainly apply opposing strategies to head classes and tail classes-those classes within LT datasets that have a limited number of samples. For instance, methods proposed by Cao et al. (2019); Khan et al. (2019); Menon et al. (2020) deliberately reduce the model output for head classes while augmenting the output for tail classes by adding constants. These approaches typically lead to improved accuracy for tail classes at the expense of reduced accuracy for head classes. Benz et al. (2021) noted similarities between the fairness issue in LT classification and that in adversarial training. They corresponded the head and tail classes in LT classification with the easy and hard classes in adversarial training, respectively.

3D point cloud semantic segmentation is one of the fundamental tasks for environmental understanding. Although significant progress has been made in recent years, the performance of classes with few examples or few points is still far from satisfactory. In this paper, we propose a novel multi-to-single knowledge distillation framework for the 3D point cloud semantic segmentation task to boost the performance of those hard classes. Instead of fusing all the points of multi-scans directly, only the instances that belong to the previously defined hard classes are fused. To effectively and sufficiently distill valuable knowledge from multi-scans, we leverage a multilevel distillation framework, i.e., feature representation distillation, logit distillation, and affinity distillation. We further develop a novel instance-aware affinity distillation algorithm for capturing high-level structural knowledge to enhance the distillation efficacy for hard classes. Finally, we conduct experiments on the SemanticKITTI dataset, and the results on both the validation and test sets demonstrate that our method yields substantial improvements compared with the baseline method. The code is available at \Url{https://github.com/skyshoumeng/M2SKD}.

Adversarial training has been widely acknowledged as the most effective method to improve the adversarial robustness against adversarial examples for Deep Neural Networks (DNNs). So far, most existing works focus on enhancing the overall model robustness, treating each class equally in both the training and testing phases. Although revealing the disparity in robustness among classes, few works try to make adversarial training fair at the class level without sacrificing overall robustness. In this paper, we are the first to theoretically and empirically investigate the preference of different classes for adversarial configurations, including perturbation margin, regularization, and weight averaging. Motivated by this, we further propose a \textbf{C}lass-wise calibrated \textbf{F}air \textbf{A}dversarial training framework, named CFA, which customizes specific training configurations for each class automatically. Experiments on benchmark datasets demonstrate that our proposed CFA can improve both overall robustness and fairness notably over other state-of-the-art methods. Code is available at \url{https://github.com/PKU-ML/CFA}.

Training accurate deep neural networks (DNNs) in the presence of noisy labels is an important and challenging task. Though a number of approaches have been proposed for learning with noisy labels, many open issues remain. In this paper, we show that DNN learning with Cross Entropy (CE) exhibits overfitting to noisy labels on some classes ("easy" classes), but more surprisingly, it also suffers from significant under learning on some other classes ("hard" classes). Intuitively, CE requires an extra term to facilitate learning of hard classes, and more importantly, this term should be noise tolerant, so as to avoid overfitting to noisy labels. Inspired by the symmetric KL-divergence, we propose the approach of \textbf{Symmetric cross entropy Learning} (SL), boosting CE symmetrically with a noise robust counterpart Reverse Cross Entropy (RCE). Our proposed SL approach simultaneously addresses both the under learning and overfitting problem of CE in the presence of noisy labels. We provide a theoretical analysis of SL and also empirically show, on a range of benchmark and real-world datasets, that SL outperforms state-of-the-art methods. We also show that SL can be easily incorporated into existing methods in order to further enhance their performance.