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arXiv · 2512.23043

Mechanistic Evidence for Preserved-but-Misaligned Representations in Non-IID FedAvg

Muhammad Haseeb·Salaar Masood·Muhammad Abdullah Sohail·Mohammad Fatim Shoaib·Muhammad Tahir
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Abstract

Federated Averaging (FedAvg) often degrades under non-IID client data, but it remains unclear whether this degradation reflects the loss of client-learned representations or a failure to use representations that are still present. We study this question mechanistically in sparse client-trained vision models, using dense-model controls to test whether the observed effects depend on sparsity. Our analysis combines class-specific circuit discovery, linear probing of frozen representations, head-only finetuning, and sparse feature dictionaries. Across CNN and ResNet models on CIFAR-10 and Fashion-MNIST, severe label skew can drive some per-class accuracies near zero even when class-specific internal structure remains recoverable. Linear probes substantially outperform the aggregated classifier, head-only finetuning partially restores accuracy, and USAE transfer reveals a largely shared feature basis between IID and non-IID models. Together, these diagnostics suggest that, in our setting, non-IID FedAvg degradation is not fully explained by representational erasure; it also reflects misalignment between preserved internal structure and the final prediction pathway.

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Muhammad Haseeb, Salaar Masood, Muhammad Abdullah Sohail, Mohammad Fatim Shoaib, Muhammad Tahir. 2025-12-28. Mechanistic Evidence for Preserved-but-Misaligned Representations in Non-IID FedAvg. https://arxiv.org/abs/2512.23043

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