For example, in the Semeval test dataset of 100 full-length academic papers, to be described later in the evaluation section, the n-grams control has, rule satisfies and become known all have a document frequency of 1.
phrases chosen by humans as keyphrases such as Query expansion which occurs in 9 documents and as a keyphrase in 4 or language models which occurs in 12 document and again in 4 of them as key, have relatively high document frequency counts.