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.
I'm a longtime fan of Hirons' blog and the book did not disappoint. It's thorough without being a total overload and it's easy to skip over sections that might not be relevant to you. Scientific and well researched without being a medical textbook. Even though I'm very familiar with Hirons' work and this is essentially an extensive summary of it, I still learnt things and took away some great new tips and recommendations. Hirons' writing style doesn't really change from blog to book, which makes it feel like a conversation with a familiar aunt or something. Highly recommend, especially if you're new to skincare or need a bit of a rev up.