Amazon calculates a product’s star ratings using a machine learned model instead of a raw data average. The machine learned model takes into account factors including: the age of a review, helpfulness votes by customers and whether the reviews are from verified purchases.
This is one of the best books I've read recently. Not only it explains neuroscience in basic terms followwed by good examples to support, but also it gives a great vision into the future we might have. It is really mindblowing.