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Causally Motivated Personalized Federated Invariant Learning with Shortcut-Averse Information-Theoretic Regularization
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Proposes FedPIN, a causally motivated personalized federated invariant learning method that uses an information-theoretic shortcut-averse regularizer anchored on a global invariant feature extractor to learn personalized invariant features under distribution shift.
Problem
Personalized federated learning must preserve client-specific features while removing spurious correlations, but personalized features are entangled with shortcuts, so OOD generalization across client test distributions suffers.
Approach
Derives a causal signature from a heterogeneous structured causal model (personalized invariant features are conditionally dependent on global invariant features given Y, while shortcuts are not) and quantifies it as a conditional mutual information term minus an entropy term, approximated via contrastive loss and variance, with a global invariant extractor as anchor.
Key results
- FedPIN reaches 59.8% worst-case accuracy on CFMNIST with 8 clients versus 56.9 for FedSDR, and 73.8% worst-case on WaterBird with 8 clients.
- FedPIN achieves the highest average accuracy on CFMNIST, WaterBird and PACS among compared methods (e.g., 63.1 on CFMNIST 8 clients, 75.8 on WaterBird 8 clients).
- Ablation shows removing the conditional mutual information term (lambda=0) drops CMNIST worst-case accuracy from 53.6 to 20.2.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| add the FedPIN shortcut-averse invariant-learning method | personalized federated learning · CFMNIST, 8 and 80 clients | worst-case accuracy: 56.9 -> 59.8; 52.8 -> 56.4 | Helped |
| add the FedPIN shortcut-averse invariant-learning method | personalized federated learning · WaterBird, 8 and 80 clients | worst-case accuracy: 65.3 -> 73.8; 60.0 -> 67.9 | Helped |
| add the FedPIN shortcut-averse invariant-learning method | personalized federated learning · PACS, 6 and 60 clients | worst-case accuracy: 52.1 -> 55.4; 48.1 -> 52.3 | Helped |
| remove the conditional mutual information term (λ = 0) (ablation) | FedPIN · CMNIST, CFMNIST, WaterBird, and PACS | worst-case accuracy: 53.6 -> 20.2; 59.8 -> 18.2; 73.8 -> 64.0; 55.4 -> 46.1 | Hurt |
| remove the conditional mutual information term (λ = 0) (ablation) | FedPIN · CMNIST, CFMNIST, WaterBird, and PACS | average accuracy: 55.4 -> 54.5; 63.1 -> 55.7; 75.8 -> 72.8; 58.6 -> 53.0 | Hurt |
| remove the entropy term (γ = 0) (ablation) | FedPIN · CMNIST, CFMNIST, WaterBird, and PACS | worst-case accuracy: 53.6 -> 52.8; 59.8 -> 58.6; 73.8 -> 69.8; 55.4 -> 52.9 | Hurt |
| remove the entropy term (γ = 0) (ablation) | FedPIN · CMNIST, CFMNIST, WaterBird, and PACS | average accuracy: 55.4 -> 55.2; 63.1 -> 63.2; 75.8 -> 75.4; 58.6 -> 56.3 | Hurt |
| increase the number of local epochs R from 5 to 20 (ablation) | FedPIN · WaterBird | worst-case accuracy: 71.8 -> 73.8 -> 73.7 -> 73.3 | Mixed |
| increase the number of local epochs R from 5 to 20 (ablation) | FedPIN · WaterBird | average accuracy: 74.4 -> 75.8 -> 76.2 -> 76.4 | Helped |
| use local fine-tuning (GM-FT) instead of the proposed personalized invariant objective (ablation) | FedPIN global invariant model · CMNIST, CFMNIST, WaterBird, and PACS | worst-case accuracy: 47.5 -> 15.1; 48.2 -> 10.7; 63.4 -> 62.8; 47.2 -> 45.5 | Hurt |
| use L2-norm regularization (GM-L2) instead of the proposed personalized invariant objective (ablation) | FedPIN global invariant model · CMNIST, CFMNIST, WaterBird, and PACS | worst-case accuracy: 47.5 -> 46.9; 48.2 -> 48.6; 63.4 -> 64.9; 47.2 -> 48.2 | Mixed |
Surprising findings
- Standard local fine-tuning and L2 regularization on the global invariant model reduce OOD accuracy (e.g., GM-FT worst-case on CMNIST 15.1 vs GM 47.5), indicating personalization entangles with shortcuts.
Limitations
- Theoretical guarantees rely on assumed SCM and Markov property; results depend on hyperparameters lambda, gamma chosen appropriately.
- Experiments are on small-scale image benchmarks with simulated clients and only ResNet-18 or small MLP backbones.
Experimental setup
Datasets: CMNIST CFMNIST WaterBird PACS
Benchmarks: CMNIST CFMNIST WaterBird PACS
Metrics: worst-case test accuracy average test accuracy
Baselines: FedAvg DRFA FedSR FedIIR pFedMe Ditto FTFA FedRep FedRoD FedPAC FedSDR
Builds on: IRM FedIIR FedSDR Ditto FedRep
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