  In this paper, we introduce the Eval4NLP-2021shared task on explainable
quality estimation. Given a source-translation pair, this shared task requires
not only to provide a sentence-level score indicating the overall quality of
the translation, but also to explain this score by identifying the words that
negatively impact translation quality. We present the data, annotation
guidelines and evaluation setup of the shared task, describe the six
participating systems, and analyze the results. To the best of our knowledge,
this is the first shared task on explainable NLP evaluation metrics. Datasets
and results are available at https://github.com/eval4nlp/SharedTask2021.
