A negative number? Remember that we are taking logarithms of probabilities in this scoring function, and the probabilities of word occurrence are small. The important issue is the effectiveness of the rankings produced using these scores. Table
7.3 shows the query likelihood scores for the same variations of term occurrences that were used in Table 7.2. Although the scores look very different for BM25 and QL, the rankings are similar, with the exception that the document containing 15 occurrences of “president” and 1 of “lincoln” is ranked higher than the document containing 0 occurrences of “president” and 25 occurrences of “lincoln” in the QL scores, whereas the reverse is true for BM25.