UNION: An Unreferenced Metric for Evaluating Open-ended Story Generation

Despite the success of existing referenced metrics (e.g., BLEU and MoverScore), they correlate poorly with human judgments for openended text generation including story or dialog generation because of the notorious oneto-many issue: there are many plausible outputs for the same input, which may differ substantially in literal or semantics from the limited number of given references. To alleviate this issue, we propose UNION, a learnable UNreferenced metrIc for evaluating Open-eNded story generation, which measures the quality of a generated story without any reference. Built on top of BERT, UNION is trained to distinguish human-written stories from negative samples and recover the perturbation in negative stories. We propose an approach of constructing negative samples by mimicking the errors commonly observed in existing NLG models, including repeated plots, conflicting logic, and long-range incoherence. Experiments on two story datasets demonstrate that UNION is a reliable measure for evaluating the quality of generated stories, which correlates better with human judgments and is more generalizable than existing state-of-theart metrics. * Corresponding author Leading Context Jack was at the bar. Reference By Human He noticed a phone on the floor. He was going to take it to lost and found. But it started ringing on the way. Jack answered it and returned it to the owner's friends. Sample 1 (Reasonable, B=0.29, M=0.49, U=1.00) On the way out he noticed a phone on the floor. He asked around if anybody owned it. Eventually he gave it to the bartender. They put it into their lost and found box. Sample 2 (Reasonable, B=0.14, M=0.27, U=1.00) He had a drinking problem. He kept having more beers. After a while he passed out. When he waked up, he was surprised to find that he lost over a hundred dollars. Sample 3 (Unreasonable, B=0.20, M=0.35, U=0.00) He was going to get drunk and get drunk. The bartender told him it was already time to leave. Jack started drinking. Jack wound up returning but cops came on the way home.