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.
I clicked on Kindle for a previous order; now even when I click on purchase HARD COPY and am willing to pay for it, I get KINDLE. So WARNING: don't ever click on Kindle out of curiosity because it is IRREVERSIBLE. You will never be able to get a hard copy. You can also not cancel the kindle order.
It gave me a lot of great ideas about how to create my own masks. It is very useful right now when it is hard to find masks to buy. It is easy to understand and follow. I recommend this book for anyone who needs and wants to create their own masks.