The problem with ratings are that they are not from a large enough random sample. Rating scores tell me what people that like to rate products think. I don't rate products and in general I suspect that people that are happy with a product don't really care to take the time to go out of their way to tell people about it online (that may change or be changing, I don't know). but one thing is for sure, people that dislike a product WILL go out of their way to tell everyone, thus further shifting the data set to being reviewed by people that are unhappy.

Take a random sample of the true population of the data set (everyone that has seen a movie) and not just the people that logon to rate it.