Hi All,
In mahout there is implemented method for item based Collaborative filtering
called itemsimilarity, which returns the "similarity" between each two items.
In the theory, similarity between two items should be calculated only for users
who ranked both items. During testing I realized that in mahout it works
different.
Below two examples.
Example 1. items are 11-12
In below example the similarity between item 11 and 12 should be equal 1, but
mahout output is 0.36. It looks like mahout treats null as 0.
Similarity between items:
101 102 0.36602540378443865
Matrix with preferences:
11 12
1 1
2 1
3 1 1
4 1
Example 2. items are 101-103.
Similarity between items 101 and 102 should be calculated using only ranks for
users 4 and 5, and the same for items 101 and 103 (that should be based on
theory). Here (101,103) is more similar than (101,102), and it shouldn't be.
Similarity between items:
101 102 0.2612038749637414
101 103 0.4340578302732228
102 103 0.2600070276638468
Matrix with preferences:
101 102 103
1 1 0.1
2 1 0.1
3 1 0.1
4 1 1 0.1
5 1 1 0.1
6 1 0.1
7 1 0.1
8 1 0.1
9 1 0.1
10 1 0.1
Both examples were run without any additional parameters.
Is this problem solved somewhere, somehow? Any ideas? Why null is treated as 0?
Source: http://files.grouplens.org/papers/www10_sarwar.pdf
Kind regards,
Natalia Gruszowska
