Can you talk more about your research specifically? Finding several successful books in a specifc genre might be more useful to you than determine which outsold which.
My research is pretty much based on Amazon.com. I live in Paris and there are only a few English bookstores and they are all small with limited selection.
I have read several books on my topic (France) and so I have them as a starting point. I then look on Amazon which shows me similar books and I kept following them. There are a few that I've read myself so I know they're good, but the thing is I have no idea if they are selling.
I'm worried that for example I compare my book to 3 others and in fact those three others haven't sold many copies so the publisher will think there is no market. Or maybe one of the books I didn't choose is outselling the one I did, etc.
I'm new at this so let me know if you have any suggestions. So far the Amazon sales rank is my only clue. And sometimes it's 10,000 and others it's 100,000. But that doesn't tell me that much really.. it doesn't mean one sold 10x more than the other..
-Michael