It has been interesting over the past few months to try to decode how Amazon calculates sales ranking by watching the relationship between sales of Newton’s Ark and its best seller ranking. Amazon guards its algorithm closely, saying very little about what the best seller ranking actually means or how it is calculated.
My key conclusion is that the ranking measures the rate of sales rather than total sales. That means it tells you what is popular recently, but not necessarily what has been most popular over time. That runs counter to what I always thought a best seller ranking meant – that the best selling book had sold the most copies total. But perhaps this has always been standard practice?
A sales rate based measure necessarily favors more recent works rather than classics. It also means for items with small sales volumes the ranking jumps around all over the place. I’ve noticed that two sales in a week can raise the ranking of the Kindle edition of Newton’s Ark by more than 300,000 places (it’s even worse with the paperback – once recent sale raised the ranking more than two million places!) To me that suggests the best seller ranking has a fairly low signal to noise ratio. Sure it’s still useful to distinguish between #1 and #100,000 but probably not between #75,000 and #175,000.
My other criticism is that Amazon ranks different editions of the same book separately. For example, a book that sells 100,000 copies on Kindle only would have a much higher best seller ranking than a book that sells 50,000 Kindle copies and 50,000 paperbacks. I think everyone will agree that both books are equally popular. If someone is trying to identify popular books they may wish to read, do they care what format other people are reading the books in, or just that they are reading them?
What’s my solution? Not that Amazon throw away it’s current best seller ranking but that it makes it clear what it means and also add additional measures. So for example, instead of just having one best seller ranking for a Kindle book you would have something like:
Best Seller Rankings
Kindle books – fastest selling: #102
Kindle books – most sold ever: #33,452
All formats – fastest selling: #245
All formats – most sold ever: #456,765