Evaluating Document Simplification: On the Importance of Separately
  Assessing Simplicity and Meaning Preservation
Liam Cripwell Joël Legrand Claire Gardent
Text simplification intends to make a text easier to read while preserving
its core meaning. Intuitively and as shown in previous works, these two
dimensions (simplification and meaning preservation) are often-times inversely
correlated. An overly conservative text will fail to simplify sufficiently,
whereas extreme simplification will degrade meaning preservation. Yet, popular
evaluation metrics either aggregate meaning preservation and simplification
into a single score (SARI, LENS), or target meaning preservation alone
(BERTScore, QuestEval). Moreover, these metrics usually require a set of
references and most previous work has only focused on sentence-level
simplification. In this paper, we focus on the evaluation of document-level
text simplification and compare existing models using distinct metrics for
meaning preservation and simplification. We leverage existing metrics from
similar tasks and introduce a reference-less metric variant for simplicity,
showing that models are mostly biased towards either simplification or meaning
preservation, seldom performing well on both dimensions. Making use of the fact
that the metrics we use are all reference-less, we also investigate the
performance of existing models when applied to unseen data (where reference
simplifications are unavailable).