  Communication of scientific findings to the public is important for keeping
non-experts informed of developments such as life-saving medical treatments.
However, generating readable lay summaries from scientific documents is
challenging, and currently, these summaries suffer from critical factual
errors. One popular intervention for improving factuality is using additional
external knowledge to provide factual grounding. However, it is unclear how
these grounding sources should be retrieved, selected, or integrated, and how
supplementary grounding documents might affect the readability or relevance of
the generated summaries. We develop a simple method for selecting grounding
sources and integrating them with source documents. We then use the BioLaySum
summarization dataset to evaluate the effects of different grounding sources on
summary quality. We found that grounding source documents improves the
relevance and readability of lay summaries but does not improve factuality of
lay summaries. This continues to be true in zero-shot summarization settings
where we hypothesized that grounding might be even more important for factual
lay summaries.
