N A V 7 1 2 bigram>co-occur PB 6 4 2 bigram<co-occur 1 1 0 bigram=co-occur 9 3 3 bigram>co-occur SC 4 1 1 bigram<co-occur 1 2 0 bigram=co-occur Table 2: Bigrams vs. Co-occurrences N A V PB 9 4 1 rbr>upgma 4 0 2 rbr<upgma 1 2 1 rbr=upgma SC 8 1 3 rbr>upgma 2 5 0 rbr<upgma 4 0 1 rbr=upgma Table 3: Repeated Bisections vs. UPGMA However, second order context vectors indirectly represent bigram features, and do not require an exact match between vectors in order to establish similarity.
the poor performance of bigrams in the case of first order context vectors suggests that when dealing with small amounts of data, we need to boost or enrich our bigram feature set by using some other larger training source like a corpus drawn from the Web.