  Pretraining is the preliminary and fundamental step in developing capable
language models (LM). Despite this, pretraining data design is critically
under-documented and often guided by empirically unsupported intuitions. To
address this, we pretrain 28 1.5B parameter decoder-only models, training on
data curated (1) at different times, (2) with varying toxicity and quality
filters, and (3) with different domain compositions. First, we quantify the
effect of pretraining data age. A temporal shift between evaluation data and
pretraining data leads to performance degradation, which is not overcome by
finetuning. Second, we explore the effect of quality and toxicity filters,
showing a trade-off between performance on standard benchmarks and risk of
toxic generations. Our findings indicate there does not exist a
one-size-fits-all solution to filtering training data. We also find that the
effects of different types of filtering are not predictable from text domain
characteristics. Lastly, we empirically validate that the inclusion of
heterogeneous data sources, like books and web, is broadly beneficial and
warrants greater prioritization. These findings constitute the largest set of
experiments to validate, quantify, and expose many undocumented intuitions
about text pretraining, which we hope will help support more informed
data-centric decisions in LM development.
