How to determine relative success of different, similar books published by different means?
This answer discusses how to choose between traditional print publishing, ebook publishing, self publishing, etc., and concludes by stating that
You should publish your book using the channel where books like yours are typically most successful.
That's certainly sensible advice, and probably quite apt if one's aim is to reach the widest audience possible for a given book, which would likely be why one publishes in the first place. (Things like money, fame, movie rights contracts and so on, are then guaranteed to follow... or not.)
But how to determine typical relative success rates among different types of publishing?
Sure, one can browse the catalogs of various online bookstores, looking for "books like [one's own]" and trying to compare rankings, but sales rankings are affected by much more than the kind of book and how it's published; for example, is the author well-known with an established fan base, or a newcomer that no one has heard of; how much marketing is being done for the author's works in general, and that specific book in particular; what price point is each book offered at; and so on. It's also a very labor-intensive process.
Short of talking to a publisher who has access to more detailed data of at least a subset of the market (and who will, of course, want to steer the author toward contracting with them if in their opinion the book is good, and not waste their time on it if it's not), is there a more expedient way? The larger a group of authors who would have access to the means suggested, the better.