Enumeration of Extractive Oracle Summaries
Table 5: Median number of oracle summaries and rates of reference summaries and topics with multiple oracle summaries for each data set
['[EMPTY]', 'Median single', 'Median single', 'Median multi', 'Median multi', 'Rate single', 'Rate single', 'Rate multi', 'Rate multi']
Over 80% of the reference summaries and about 60% to 90% of the topics have multiple oracle summaries. Since the \sc Rougen scores are based on the unweighted counting of n-grams, when many sentences have similar meanings, i.e., many redundant sentences, the number of oracle summaries that have the same \sc Rougen scores increases. The source documents of multiple document summarization tasks are prone to have many such redundant sentences, and the amount of oracle summaries is large.