Github user mengxr commented on the pull request:
https://github.com/apache/spark/pull/597#issuecomment-42256964
MovieLens ratings are on a scale of 1-5:
~~~
5: Must see
4: Will enjoy
3: It's okay
2: Fairly bad
1: Awful
~~~
So we should not recommend a movie if the predicted rating is less than
`3`. To map ratings to confidence scores, I would use `5 -> 2, 4 -> 1, 3 -> 0,
2 -> -1, 1 -> -2` or `5 -> 2.5, 4 -> 1.5, 3 -> 0.5, 2 -> -0.5, 1 -> -1.5`. The
latter mappings means unobserved entries are generally between `It's okay` and
`Fairly bad`.
For evaluation, the mapping should be `if (r >= 3) 1.0 else 0.0` for
MovieLens ratings, and I agree with @srowen on weighted RMSE.
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