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.
though not quite what I was looking for, was still a good read. some of the info may be a little dated but it does explain key concepts quite well. for nicer diagrams with better explanations the one to try would be