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Learning from Crowds Using a Focal Loss Function: Dealing with Imbalanced Annotations

delete2026-06-18
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J
Julián Gil-González *
D
David Cárdenas‐Peña
A
Alvaro Orozco-Gutiérrez
E
Enrique D. Guijarro-Estelles
A
Andrés Marino Álvarez-Meza
DOI:10.3390/technologies14060370delete
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Abstract

Abstract

En 中文
Obtaining high-quality labeled data for supervised learning is costly, motivating the use of crowdsourcing, which distributes the annotation process across multiple workers with varying levels of expertise. A key challenge in crowdsourced data is annotation sparsity, as each worker labels only a limited subset of instances. This sparsity can amplify class imbalance, reduce supervision for minority classes, and bias standard cross-entropy-based models toward the majority classes. To address this problem, we propose a correlated chained Gaussian process framework trained on a focal-loss-based variational objective (CCGPFL). This probabilistic framework jointly models latent ground-truth and instance-dependent annotator reliability while accounting for correlations among annotators. In addition, the focal-weighted objective mitigates the imbalance induced by sparse annotations by assigning greater importance to harder examples during training. Experiments on synthetic, semi-synthetic, and fully real multi-annotator datasets show that CCGPFL achieves competitive and often superior performance relative to state-of-the-art learning-from-crowds baselines in terms of Overall Accuracy (OA) and Area Under the ROC Curve (AUC).
Keywords:
multiple annotators
Gaussian processes
noisy labels
imbalanced data
focal loss
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universidad tecnológica de pereira
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