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As before, we avoid representing zero values explicitly by
replacing each data point by the list , where
.
A co-occurrence is represented by the quadruple ,
. Instead of one co-occurrence count , we
now have a two-way contingency table . Each
represents the number of data points where
.
Counting and storing co-occurrences can be done in the same
time as before and with a
larger amount
of memory, necessitated by the additional need to store the
(non-zero) variable values.

Journal of Machine Learning Research
2000-10-19