Difficulty–Diversity Collaborative Filtering for Data-Efficient LLM Fine-Tuning

The performance of fine-tuned language models is heavily influenced by the quality and quantity of their fine-tuning data. While scaling laws suggest that larger models benefit from more data during pretraining, the Less-is-More hypothesis highlights that downstream fine-tuning often requires only a small but high-quality dataset to effectively elicit a model's pretrained knowledge. However, identifying such premium data, particularly in terms of difficulty and diversity, typically relies on human expertise, and existing methods offer limited guidance for automatic selection from large unannotated corpora. This work presents a novel quantitative framework that formalizes the interplay between question difficulty and diversity, and introduces Difficulty-Diversity Collaborative Filtering (DDCF): an automated approach that tailors data selection to the unique characteristics of each language model via collaborative filtering. By leveraging a small seed dataset to predict correctness across a large unannotated corpus, our method reduces the annotation cost by 100-200×, while maintaining downstream performance comparable to full-corpus fine-tuning. 1 INTRODUCTION The remarkable success of Large Language Models (LLMs) in recent years (Grattafiori et al., 2024b; Yang et al., 2025b) stems largely from their ability to learn rich and generalizable representations from massive pretraining corpora. To further enhance capabilities of these models on downstream tasks, supervised fine-tuning (SFT) has become a popular approach (Wei et al., 2022; Chung et al., 2024) . However, SFT typically involves fine-tuning pretrained models on large-scale, human-annotated instruction datasets, often comprising hundreds of thousands of examples. Despite its effectiveness, fine-tuning on such large datasets presents several challenges. First, data collection and model training incur substantial computational costs. Second, updating a model on a new large corpus may cause catastrophic forgetting, where continual learning of new tasks degrades performance on previously acquired knowledge (Biderman et al., 2024; Wang et al., 2024) . Third, scaling up the dataset often leads to over-representation of common patterns, reducing diversity and underrepresenting rare but important examples (Kim et al., 2022; Zhang et al., 2025a). Recently, the Less-is-More hypothesis (Zhou et al., 2023; Ye et al., 2025; Dohmatob et al., 2025) has suggested that downstream task adaptation can be achieved through minimal supervision, where the model primarily learns task-specific formatting or styles to reveal knowledge already encoded during pretraining. Empirical studies have shown that fine-tuning on just a few carefully selected examples sometimes outperforms naively using vast annotated corpora (Zhou et al., 2023; Ye et al., 2025; Muennighoff et al., 2025) . Furthermore, theoretical analysis (Dohmatob et al., 2025) demonstrates that, when the base model is strong, selecting harder examples offers a provable advantage. However, such curated datasets often rely on evolving human expertise, making them labor-intensive, inflexible, and inconvenient to adapt to new models or tasks. While recent efforts have explored automated methods to improve data quality (Xia et al., 2024; Yang et al., 2024b), the automatic selection without annotated output responses remains an open