To calculate the overall star rating and percentage breakdown by star, we don’t use a simple average. Instead, our system considers things like how recent a review is and if the reviewer bought the item on Amazon. It also analyses reviews to verify trustworthiness.
Don't buy this book. You will get frustrated, just after reading few pages. There are so many errors, solutions & questions contradicting each other. I think the author or editor was drunk before making this piece of crap.