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Analytical Correction for Subsampling Bias in Drifting Models
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Shows the minibatch softmax-weighted centroid in drifting models has an O(1/n) self-normalization bias and proposes Analytical Bias Correction (ABC), a closed-form plug-in fix that reduces bias to O(1/n^2) and improves CIFAR-10 FID at small n.
Problem
Drifting models approximate softmax-weighted attractive and repulsive centroids over full data and generator distributions with minibatches of size n; the self-normalized minibatch centroid is biased, and this bias degrades generation quality at small n.
Approach
Derive the leading O(1/n) bias of the self-normalized softmax centroid and correct it with a closed-form plug-in term sum_i alpha_i^2 (T_n - y_i) computed from the same minibatch, giving a corrected centroid that is still a convex combination of minibatch points.
Key results
- On CIFAR-10, ABC improves last-5-average FID at every tested n, e.g. 7.31 to 6.40 at n=8 and 4.34 to 3.89 at n=32.
- At n=8 with FID <= 10, ABC needs about 3.38M positive samples versus 8.19M for the standard estimator.
- Toy experiments show bias slopes of about -1 for the standard centroid and -2 for ABC on log-log axes.
Surprising findings
- Lowering the kernel temperature cannot substitute for the correction, per the compensation analysis.
- Epoch wall time with torch.compile is within measurement noise of the standard method (37.0s vs 38.0s fused).
Limitations
- Gains on CIFAR-10 are small at n=16 and n=64 and within seed variance; best FID at n=64 is worse for ABC (3.40 vs 3.24).
- Bias theorems assume bounded kernel weights and features, and the higher-order toy results show ABC and standard nearly overlapping at small n.
- Evaluation limited to a small DriftDiT-Small model on CIFAR-10 with three seeds.
Experimental setup
Datasets: CIFAR-10 CIFAR-100
Benchmarks: CIFAR-10 CIFAR-100
Metrics: FID wall-clock time per epoch peak GPU memory bias norm
Baselines: Standard minibatch centroid jackknife bootstrap correction BR-SNIS
Builds on: drifting models delta method for self-normalized estimators DriftDiT
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