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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