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Consistent algorithms for multi-label classification with macro-at-$k$ metrics
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Gives a population-utility framework for macro-averaged multi-label metrics under a budget of exactly k predicted labels per instance, proves the optimal classifier form, and proposes a consistent Frank-Wolfe algorithm for it.
Problem
Optimizing complex macro-averaged multi-label metrics (e.g., macro-F1, macro-precision, macro-recall) when predictions must contain exactly k labels per instance; the budget constraint couples the per-label binary problems.
Approach
Shows the optimal budgeted classifier for a linear utility is top-k over affine transforms of label marginals, extends this to general (non-linear) confusion-tensor metrics via a compactness argument, and optimizes them with a Frank-Wolfe procedure over feasible confusion tensors that is statistically consistent.
Key results
- Frank-Wolfe classifiers achieve the best result on the target macro measure in most cases across Mediamill, Flickr, and RCV1-x at k in {3,5,10}.
- On the largest dataset (AmazonCat-13K), where most labels have fewer than 10 positives, simple power-law weighting heuristics sometimes outperform the Frank-Wolfe approach.
- The optimal macro-recall strategy recovered by the Frank-Wolfe method matches the closed-form solution in all cases.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Use Frank-Wolfe to optimize macro-at-k metrics | Multi-label classification benchmarks · Mediamill, Flickr, RCV1, and AmazonCat | target macro-at-k metric | Helped |
| Use Frank-Wolfe macro-at-k classifiers | Multi-label classification benchmarks · Mediamill, Flickr, RCV1, and AmazonCat | instance-wise measures | Mixed |
| Use Frank-Wolfe instead of simple power-law weighting | Macro-at-k classification · AmazonCat, the largest dataset | macro-at-k metric | Hurt |
| Use focal loss or asymmetric loss instead of binary cross-entropy | Multi-label classification · Reported benchmark tasks | macro measures | No clear effect |
| Use focal loss or asymmetric loss instead of binary cross-entropy | Multi-label classification · Reported benchmark tasks | instance-wise metrics | No clear effect |
| Use the Frank-Wolfe macro-recall classifier | Macro recall optimization · All reported cases | macro recall | No clear effect |
Surprising findings
- Frank-Wolfe optimization for macro measures can sacrifice instance-wise metrics such as precision@k, while improving the targeted macro metric.
- Focal and asymmetric losses preserve instance-wise performance but give only small gains on macro measures.
Limitations
- On the largest dataset with many rare labels, estimates of marginals and confusion matrices are inaccurate, so Frank-Wolfe underperforms simple heuristics.
- Consistency results rely on assumptions such as absolute continuity of label marginals and differentiable, strictly monotone metrics.
Experimental setup
Datasets: Mediamill Flickr RCV1-x AmazonCat-13K
Benchmarks: Mediamill Flickr RCV1-x AmazonCat-13K
Metrics: instance-wise precision@k instance-wise recall@k macro precision@k macro recall@k macro F1@k
Baselines: top-k BCE power-law weighted top-k log-weighted top-k focal loss asymmetric loss macro-recall optimal rule
Builds on: Frank-Wolfe algorithm multi-class consistent classification with constraints (Narasimhan et al.) Madow's sampling scheme focal loss asymmetric loss
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