However, when you're talking about dozens of movies, all you need is a correlation. Our algorithm is powerful enough to tolerate a large amount of noise. If you read the paper, we were able to match up users between imdb and netflix with a very high level of confidence, in the sense that the best match was 15-30 standard deviations away from the second best match. In statistics terms, that's a insanely close match.
Variables don't; constants aren't.