Friends in Unexpected Places: Enhancing Local Fairness in Federated Learning through Clustering (2407.19331v3)

Abstract: Federated Learning (FL) has been a pivotal paradigm for collaborative training of machine learning models across distributed datasets. In heterogeneous settings, it has been observed that a single shared FL model can lead to low local accuracy, motivating personalized FL algorithms. In parallel, fair FL algorithms have been proposed to enforce group fairness on the global models. Again, in heterogeneous settings, global and local fairness do not necessarily align, motivating the recent literature on locally fair FL. In this paper, we propose new FL algorithms for heterogeneous settings, spanning the space between personalized and locally fair FL. Building on existing clustering-based personalized FL methods, we incorporate a new fairness metric into cluster assignment, enabling a tunable balance between local accuracy and fairness. Our methods match or exceed the performance of existing locally fair FL approaches, without explicit fairness intervention. We further demonstrate (numerically and analytically) that personalization alone can improve local fairness and that our methods exploit this alignment when present.

• The paper demonstrates that personalization can inadvertently enhance group fairness while improving individual model accuracy in FL.
• It introduces fairness-aware federated clustering algorithms, Fair-FCA and Fair-FL+HC, that balance local performance and fairness.
• Statistical analysis reveals that personalized FL models align accuracy and fairness, reducing overfitting toward majority data.

Enhancing Group Fairness in Federated Learning Through Personalization: A Detailed Analysis

This paper investigates the interplay between personalization and group fairness in Federated Learning (FL), a decentralized learning paradigm that retains data privacy by enabling distributed data training. In typical FL, models are trained collaboratively across a diverse set of clients to build a robust global model. However, these models often fall short in customization for individual clients and can inadvertently neglect the data disparities among various demographic groups, leading to systemic biases. Addressing these challenges, the authors explore how personalization techniques, which are predominantly designed to enhance local accuracy, can simultaneously improve fairness, mitigating bias across different groups.

Main Contributions

1. Unintended Fairness Benefits: The authors demonstrate through extensive numerical experiments that personalization inadvertently enhances fairness. Surprisingly, techniques primarily aimed at optimizing local accuracy also contribute favorably to reducing fairness disparities. Key experiments—utilizing datasets like the "Adult" and "Retiring Adult"—illustrate the potential for a dual benefit in both accuracy and fairness, suggesting statistical diversity and computational alignment as contributing factors.
2. Fairness-Aware Federated Clustering Algorithms: Inspired by the unintended fairness benefits observed, the paper proposes two new algorithms: Fair-FCA and Fair-FL+HC. These are designed to weave a fairness metric into the client clustering process, optimizing both local model accuracy and fairness. By incorporating fairness considerations into the clustering mechanism, these algorithms achieve a tunable balance, providing a preferable trade-off between fairness and accuracy.
3. Statistical and Computational Insights: The research supports its findings with statistical analysis and computational insights. Under certain conditions, the paper posits that personalized and clustered FL models better align accuracy and fairness objectives, offering empirical evidence that personalization reduces overfitting tendencies to the majority data.

Implications and Future Directions

The implications of this work are significant; they indicate a path forward where federated personalization not only addresses client-specific accuracy needs but also promotes social fairness without additional fairness constraints. This discovery opens new avenues for developing fairness-centric personalized algorithms that can adaptively balance dual objectives within FL frameworks.

From a theoretical perspective, the paper's analytical support suggests that the conditions under which personalization improves fairness can inform the design of future personalized FL systems. Future work could extend these insights to other classes of personalized FL methods beyond clustering-based approaches and investigate leveraging these findings in real-world applications where fairness is crucial, such as in healthcare and finance.

Overall, this paper provides a structured examination of personalization's role in advancing fairness in FL, offering a compelling narrative supported by empirical data and newly proposed methodologies. Through its dual-focused algorithms, it sets a precedent for the integration of fairness and personalization, fostering a fairer and more efficient federated learning paradigm.