I am passionate about the science of language models: developing methods—also drawing from econometrics—to study the effect of training data on models’ behaviour. Currently, I focus on active learning, data valuation, and memorisation estimation.
See the up-to-date list of publications on my Google Scholar page.
Understanding memorisation in language models has practical and societal implications, e.g., studying models’ training dynamics or preventing copyright infringements.Prior work defines memorisation as the causal effect of training with an instance on the model’s ability to predict that instance. This definition relies on a counterfactual: the ability to observe what would have happened had the model not seen that instance.Existing methods struggle to provide computationally efficient and accurate estimates of this counterfactual. Further, they often estimate memorisation for a model architecture rather than for a specific model instance. This paper fills an important gap in the literature, proposing a new, principled, and efficient method to estimate memorisation based on the difference-in-differences design from econometrics. Using this method, we characterise a model’s memorisation profile–its memorisation trends across training–by only observing its behaviour on a small set of instances throughout training.In experiments with the Pythia model suite, we find that memorisation (i) is stronger and more persistent in larger models, (ii) is determined by data order and learning rate, and (iii) has stable trends across model sizes, thus making memorisation in larger models predictable from smaller ones.
@inproceedings{lesci-etal-2024-causal,title={Causal Estimation of Memorisation Profiles},author={Lesci, Pietro and Meister, Clara and Hofmann, Thomas and Vlachos, Andreas and Pimentel, Tiago},year={2024},month=aug,booktitle={Proceedings of the 62nd Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers)},publisher={Association for Computational Linguistics},address={Bangkok, Thailand},pages={15616--15635},doi={10.18653/v1/2024.acl-long.834},url={https://aclanthology.org/2024.acl-long.834},editor={Ku, Lun-Wei and Martins, Andre and Srikumar, Vivek},}
AnchorAL: Computationally Efficient Active Learning for Large and Imbalanced Datasets
Lesci, Pietro, and Vlachos, Andreas
In Proceedings of the 2024 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies (Volume 1: Long Papers) Jun 2024
Active learning for imbalanced classification tasks is challenging as the minority classes naturally occur rarely. Gathering a large pool of unlabelled data is thus essential to capture minority instances. Standard pool-based active learning is computationally expensive on large pools and often reaches low accuracy by overfitting the initial decision boundary, thus failing to explore the input space and find minority instances. To address these issues we propose AnchorAL. At each iteration, AnchorAL chooses class-specific instances from the labelled set, or anchors, and retrieves the most similar unlabelled instances from the pool. This resulting subpool is then used for active learning. Using a small, fixed-sized subpool AnchorAL allows scaling any active learning strategy to large pools. By dynamically selecting different anchors at each iteration it promotes class balance and prevents overfitting the initial decision boundary, thus promoting the discovery of new clusters of minority instances. Experiments across different classification tasks, active learning strategies, and model architectures AnchorAL is (i) faster, often reducing runtime from hours to minutes, (ii) trains more performant models, (iii) and returns more balanced datasets than competing methods.
@inproceedings{lesci-vlachos-2024-anchoral,title={{AnchorAL}: {C}omputationally Efficient Active Learning for Large and Imbalanced Datasets},author={Lesci, Pietro and Vlachos, Andreas},year={2024},month=jun,booktitle={Proceedings of the 2024 Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies (Volume 1: Long Papers)},publisher={Association for Computational Linguistics},address={Mexico City, Mexico},pages={8445--8464},doi={https://doi.org/10.48550/arXiv.2404.05623},url={https://aclanthology.org/2024.naacl-long.467},editor={Duh, Kevin and Gomez, Helena and Bethard, Steven}}
Tending Towards Stability: Convergence Challenges in Small Language Models
Diehl Martinez, Richard, Lesci, Pietro, and Buttery, Paula
In (to appear) Proceedings of the 2024 Conference on Empirical Methods in Natural Language Processing Nov 2024
Increasing the number of parameters in language models is a common strategy to enhance their performance. However, smaller language models remain valuable due to their lower operational costs. Despite their advantages, smaller models frequently underperform compared to their larger counterparts, even when provided with equivalent data and computational resources. Specifically, their performance tends to degrade in the late pretraining phase. This is anecdotally attributed to their reduced representational capacity. Yet, the exact causes of this performance degradation remain unclear. We use the Pythia model suite to analyse the training dynamics that underlie this phenomenon. Across different model sizes, we investigate the convergence of the Attention and MLP activations to their final state and examine how the effective rank of their parameters influences this process. We find that nearly all layers in larger models stabilise early in training—within the first 20%—whereas layers in smaller models exhibit slower and less stable convergence, especially when their parameters have lower effective rank. By linking the convergence of layers’ activations to their parameters’ effective rank, our analyses can guide future work to address inefficiencies in the learning dynamics of small models.
2023
Diable: Efficient Dialogue State Tracking as Operations on Tables
Sequence-to-sequence state-of-the-art systems for dialogue state tracking (DST) use the full dialogue history as input, represent the current state as a list with all the slots, and generate the entire state from scratch at each dialogue turn. This approach is inefficient, especially when the number of slots is large and the conversation is long. We propose Diable, a new task formalisation that simplifies the design and implementation of efficient DST systems and allows one to easily plug and play large language models. We represent the dialogue state as a table and formalise DST as a table manipulation task. At each turn, the system updates the previous state by generating table operations based on the dialogue context. Extensive experimentation on the MultiWoz datasets demonstrates that Diable (i) outperforms strong efficient DST baselines, (ii) is 2.4x more time efficient than current state-of-the-art methods while retaining competitive Joint Goal Accuracy, and (iii) is robust to noisy data annotations due to the table operations approach.
@inproceedings{lesci-etal-2023-diable,title={Diable: Efficient Dialogue State Tracking as Operations on Tables},author={Lesci, Pietro and Fujinuma, Yoshinari and Hardalov, Momchil and Shang, Chao and Benajiba, Yassine and Marquez, Lluis},year={2023},month=jul,booktitle={Findings of the Association for Computational Linguistics: ACL 2023},publisher={Association for Computational Linguistics},address={Toronto, Canada},pages={9697--9719},doi={10.18653/v1/2023.findings-acl.615},url={https://aclanthology.org/2023.findings-acl.615},editor={Rogers, Anna and Boyd-Graber, Jordan and Okazaki, Naoaki}}