A frequent question in the comments is:
How were books classified as “Indie-Published,” “Small/Medium Publisher,” “Amazon Imprint Published,” “Big Five Published,” or “Uncategorized Single-Author Publisher”?
1) The Big-5 Published books were easy to separate out, no matter what imprint they were published under, by checking the “Sold By” line in the Amazon Product Details, which listed one of: Random House, Penguin, Hachette, Macmillan, HarperCollins, or Simon & Shuster as seller.
2) Amazon’s Publishing imprints were also easy to separate out, as there are relatively few of them. You can find a list of them here.
3) If multiple author names used the same listed Publisher, it wasn’t an Amazon Publishing imprint and the book’s “Sold By” wasn’t one of the Big-5, it was considered a Small/Medium Publisher. A lot of these might indeed be Indie Publishers, but we wanted to be conservative and err on the side of understating–rather than overstating–Indie numbers.
4) If no Publisher at all was listed under Product Details, the book was considered Indie-Published.
5) If the full name of the author was included in the Publisher name, the book was considered Indie-Published.
6) The remaining books, whose publishers represented only a single author name, were initially grouped under Uncategorized Single-Author Publisher, and sorted by revenue. Then we rolled up our sleeves.
7) Going down the list one by one, we Googled the publisher names and author names. We were able to classify hundreds of them as the self-publishing imprints of known Indie authors in this manner. Many of these were already known to us… for example: Broad Reach Publishing (Hugh), Laree Bailey Press (H.M. Ward), Reprobatio Inc. (Russell Blake), etc. We started from the biggest earners and went down, until the names became too obscure to find and we ran out of energy and time, and none of the remaining Uncategorized Single-Author Publishers individually accounted for a significant chunk of revenue.
So the vast majority of the remaining Uncategorized Single-Author Publishers are most likely “Indies in disguise.” But there are also a few examples of poor-selling imprints of small and medium traditional publishers in the mix, so again we didn’t want to overstate Indie market share by lumping them all in with the Indies.