  With the advent of large language models, methods for abstractive
summarization have made great strides, creating potential for use in
applications to aid knowledge workers processing unwieldy document collections.
One such setting is the Civil Rights Litigation Clearinghouse (CRLC)
(https://clearinghouse.net),which posts information about large-scale civil
rights lawsuits, serving lawyers, scholars, and the general public. Today,
summarization in the CRLC requires extensive training of lawyers and law
students who spend hours per case understanding multiple relevant documents in
order to produce high-quality summaries of key events and outcomes. Motivated
by this ongoing real-world summarization effort, we introduce Multi-LexSum, a
collection of 9,280 expert-authored summaries drawn from ongoing CRLC writing.
Multi-LexSum presents a challenging multi-document summarization task given the
length of the source documents, often exceeding two hundred pages per case.
Furthermore, Multi-LexSum is distinct from other datasets in its multiple
target summaries, each at a different granularity (ranging from one-sentence
"extreme" summaries to multi-paragraph narrations of over five hundred words).
We present extensive analysis demonstrating that despite the high-quality
summaries in the training data (adhering to strict content and style
guidelines), state-of-the-art summarization models perform poorly on this task.
We release Multi-LexSum for further research in summarization methods as well
as to facilitate development of applications to assist in the CRLC's mission at
https://multilexsum.github.io.
