In line with the general categorization of hallucination in Section 2.1, we give two concrete
hallucination examples in GQA in Table 1. The sources of both questions are Wikipedia web pages.
For the first question, “dow jones industrial average please?”, the generated answer “index of 30
major U.S. stock indexes” conflicts with the statement “of 30 prominent companies listed on stock
exchanges in the United States” from Wikipedia. So we categorize it as an intrinsic hallucination. For

ACM Comput. Surv., Vol. 1, No. 1, Article . Publication date: February 2022.

Survey of Hallucination in Natural Language Generation

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the second example, the sentences “The definition of a Sadducee is a person who acts in a deceitful or
duplicitous manner. An example of a Sadduceee is a politician who acts deceitfully in order to gain
political power” in the generated answer can not be verified from the source documents; thus, we
categorize it as an extrinsic hallucination.
SurveyofHallucinationinNatural Language Generation