Diagonals in Item-Item Similarity Matrix Set to Zero

[r-gavin]

Hello,

I have been running some tests with a catalog of 50 unique items and a usage file of 200 unique users. After training the model, I can construct a 'user recommendation matrix' from the recommendations (k = 50) for each user. From this, and believing that SAR builds the user affinity matrix in the way that I think it does, I am able to fully reconstruct the item-item similarity matrix.

When I reconstruct the item-item similarity matrix, I find that ALL of the diagonal values are set to zero. This behavior is observed with cooccurrence, jaccard, and lift.

Please tell me if this is the desired behavior, and if so, why. I can not find any reason to set these diagonal elements (essentially a measure of an items frequency) to zero.

Thanks,
Ryan