[Thinking Cap] on collaborative filtering...

[Subbarao Kambhampati <rao@asu.edu>]

Qn 1. We considered using the user-item matrix to find most similar users to the current user, and use them to predict the rating of new items for the current user. What if we decided instead to focus on items, and figure out items that seem to be most "similar" to each other in terms of the users who buy it.
   1.a. What techniques, that we have already seen, can be used to find the items most similar to each other?
    1.b. If you use those techniques to find, for each item, k most similar items, then how can we use this information to do ratings prediction for the current user on an item she hasn't yet rated?
      1.c. how does this item-centered analysis compare to user-centered one in terms of justifying the recommendations?

Qn 2. the User-item matrix has some similarities to the document-term matrix. In the case of d-t matrix, we saw that latent semantic indexing can be quite useful in finding "latent regularities" (e..g the "size" factor of the fishes, or the "database-oriented" vs. "statistics oriented" factors of the db example). 
  2.a. Do you think LSI style analysis could be useful in improving collaborative filtering?
  2.b. What are some of the most immediate problems you run into when you try to do LSI-style analysis on user-item matrix (as against d-t matrix)?

Rao
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Optional question: What does the following parable have to do with a CSE494/598 student who keeps asking the instructor "is there any way I can get extra credit to improve my grade?" ;-)

A man stood on top of his roof in the middle of one of the worst floods 
ever recorded. The water was rising around his feet so he looked up at 
the black sky and prayed.
"Mighty God please save me!"
An hour passed and the water was already over his feet when a man in a 
fishing boat came and called for him to get in. The man refused saying.
"No, I am waiting for God to save me." 
The man in the boat waited as long as he could then left the man on his 
roof. Standing soaked on the roof as the waters rose he stared at the 
sky waiting for God to save him. Then a rescue boat pulled up beside his
 house and threw him a rope but again the man refused.
"No, I am waiting for God to save me"
Hours passed and the man had to cling to his chimny to avoid being 
washed away. He thought he was going to drown when he saw a bright light
 overhead he though God had come to save him this time. Then he heard a 
loudspeeker and realized it was a hellicopter. It dropped a ladder but 
again the man refused.
"No! God will save me!" the man clung to the chimney  until the water 
was over his head and he drowned.
A moment later he stood before God weeping.
"Mighty God, I prayed and prayed why didnt you save me?" 
God said "YOW. I sent you a fishing boat, a rescue boat *and* a helecopter to save you. What the heck are you doing here?"