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.
Emily Oster is amazing, and just like with 'Expecting Better,' this book is fantastic. She lays out all sorts of issues that you'll come up against, includes citations for her sources, and gives you the scientific facts, not biased studies. It's a really amazing book that all new parents should read.