  Automatic evaluation for sentence simplification remains a challenging
problem. Most popular evaluation metrics require multiple high-quality
references -- something not readily available for simplification -- which makes
it difficult to test performance on unseen domains. Furthermore, most existing
metrics conflate simplicity with correlated attributes such as fluency or
meaning preservation. We propose a new learned evaluation metric (SLE) which
focuses on simplicity, outperforming almost all existing metrics in terms of
correlation with human judgements.
