  A key challenge for abstractive summarization is ensuring factual consistency
of the generated summary with respect to the original document. For example,
state-of-the-art models trained on existing datasets exhibit entity
hallucination, generating names of entities that are not present in the source
document. We propose a set of new metrics to quantify the entity-level factual
consistency of generated summaries and we show that the entity hallucination
problem can be alleviated by simply filtering the training data. In addition,
we propose a summary-worthy entity classification task to the training process
as well as a joint entity and summary generation approach, which yield further
improvements in entity level metrics.
