The cooccurrence counts themselves form a symmetric matrix. (A'A)' = A'A because of the way that matrix multiplication works.
The filtering for anomalous cooccurrence that sparsifies the cooccurrence can introduce asymmetry as you point out. The most prominent time that I saw this in practice was in music recommendations where a fair number of artists linked to high profile bands such as the Beatles, but the reverse link did not survive the filtering. You can enforce bi-directionality, but I have usually found that the asymmetry isn't a problem and often accords with intuitions about the field. On Wed, Sep 30, 2009 at 12:42 AM, Shashikant Kore <[email protected]>wrote: > Some time back I had thought about this idea. But, I sensed one > potential problem with this approach. The resulting co-occurrence will > be bi-directional. For document this property is fine, but for terms, > it may not be desirable in some cases. > -- Ted Dunning, CTO DeepDyve
