Given a reference text about IMDB, tell me how it rates movies.
IMDB allows users to submit their reviews and ratings.  As the most extensive internet movie database and the 54th most visited website across the planet, the scale of reviews and ratings collected for movies is massive.  IMDB uses various mathematical calculations to determine the rating of movies.  Many users decide whether to spend time and money on a film solely on ratings from IMDB.  Because of IMDB's scale and impact, it is now a subsidiary of Amazon.