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Deduplicating Training Data Makes Language Models Better

Andrew Nystrom
Chiyuan Zhang
Chris Callison-Burch
Nicholas Carlini
(2022) (to appear)
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As large language models scale up, researchers and engineers have chosen to use larger datasets of loosely-filtered internet text instead of curated texts. We find that existing NLP datasets are highly repetitive and contain duplicated examples. For example, there is an example in the training dataset C4 that has over 200,000 near duplicates. As a whole, we find that 1.68% of the C4 are near-duplicates. Worse, we find a 1% overlap between the training and testing sets in these datasets. Duplicate examples in training data inappropriately biases the distribution of rare/common sequences. Models trained with non-deduplicated datasets are more likely to generate ``memorized" examples. Additionally, if those models are used for downstream applications, such as scoring likelihoods of given sequences, we find that models trained on non-deduplicated and deduplicated datasets have a difference in accuracy of on average TODO.