  Natural Language Generation (NLG) typically involves evaluating the generated
text in various aspects (e.g., consistency and naturalness) to obtain a
comprehensive assessment. However, multi-aspect evaluation remains challenging
as it may require the evaluator to generalize to any given evaluation aspect
even if it's absent during training. In this paper, we introduce X-Eval, a
two-stage instruction tuning framework to evaluate the text in both seen and
unseen aspects customized by end users. X-Eval consists of two learning stages:
the vanilla instruction tuning stage that improves the model's ability to
follow evaluation instructions, and an enhanced instruction tuning stage that
exploits the connections between fine-grained evaluation aspects to better
assess text quality. To support the training of X-Eval, we collect
AspectInstruct, the first instruction tuning dataset tailored for multi-aspect
NLG evaluation spanning 27 diverse evaluation aspects with 65 tasks. To enhance
task diversity, we devise an augmentation strategy that converts human rating
annotations into diverse forms of NLG evaluation tasks, including scoring,
comparison, ranking, and Boolean question answering. Extensive experiments
across three essential categories of NLG tasks: dialogue generation,
summarization, and data-to-text coupled with 21 aspects in meta-evaluation,
demonstrate that our X-Eval enables even a lightweight language model to
achieve a comparable if not higher correlation with human judgments compared to
the state-of-the-art NLG evaluators, such as GPT-4.
