Over the past few months, Amazon.com has changed the way their sales rank has been calculated, bringing havoc to the author community who often use this value as a gage as to how well their books are doing.
Since Amazon does not disclose how the ranks are computed, authors were left clueless as to what the new rankings (especially considering their volatility) implied. Morris Rosenthal has spend the last month gathering some data points and has made an analysis of the new system. Read the full details HERE.
With the old system, the sales rank was determined by a combination of immediate sales as well as some historical sales figures. So it was not uncommon for a title that suddently sold a few copies in a day to have it's rank dramatically jump due to the sale but to slowly slip back down to it's "historical" level over the next few days.
With the new system, it seems that the ranking is determined mostly on the rate of sale of a book rather than a combination of total sales + immediate sales. Although an immediate sale may spike the ranking, it will slip back in a matter of hours instead of days.
What does this mean to authors? It means that the ranking does not figure how well your book has done overall at amazon but at which rate the book it selling, thus a book that has a big boom for a few weeks will have a good ranking but will slip back quickly when sales begin to taper off.
For example, a book with a ranking of 1,000 means it sells approximately 90 copies per week. Where as a book with a ranking of 500,000 probably sells around one copy per week.
As i have mentioned to the complete details on the amazon ranking system, visit the Foner Books website. This website has tons of great information for both authors and self-publishers!