  For most natural language processing tasks, the dominant practice is to
finetune large pretrained transformer models (e.g., BERT) using smaller
downstream datasets. Despite the success of this approach, it remains unclear
to what extent these gains are attributable to the massive background corpora
employed for pretraining versus to the pretraining objectives themselves. This
paper introduces a large-scale study of self-pretraining, where the same
(downstream) training data is used for both pretraining and finetuning. In
experiments addressing both ELECTRA and RoBERTa models and 10 distinct
downstream classification datasets, we observe that self-pretraining rivals
standard pretraining on the BookWiki corpus (despite using around
$10\times$--$500\times$ less data), outperforming the latter on $7$ and $5$
datasets, respectively. Surprisingly, these task-specific pretrained models
often perform well on other tasks, including the GLUE benchmark. Besides
classification tasks, self-pretraining also provides benefits on structured
output prediction tasks such as span based question answering and commonsense
inference, often providing more than $50\%$ of the performance boosts provided
by pretraining on the BookWiki corpus. Our results hint that in many scenarios,
performance gains attributable to pretraining are driven primarily by the
pretraining objective itself and are not always attributable to the use of
external pretraining data in massive amounts. These findings are especially
relevant in light of concerns about intellectual property and offensive content
in web-scale pretraining data.
