Amazon calculates a product’s star ratings based on a machine learned model instead of a raw data average. The model takes into account factors including the age of a rating, whether the ratings are from verified purchasers, and factors that establish reviewer trustworthiness.
Great book, I bought it for my friend, she was getting ready to her job interview with this book. Lots of great examples. Only the paper is a bit too thin, easy to tear a page as the book itself is pretty heavy.