* here's a long Facebook essay arguing that the Batgirl title lost sales momentum because of an editorial choice to kill a cover out of step with the current direction of the comics. I'm not sure how opting away from a jarringly different cover is best seen as a break in momentum, and don't understand the confidence in the causation elements of these numbers more generally, but it's intriguing to me that comics culture still insists on having this debate.
* the latter two posts from Library of American Comics about strip advertising have posted: here and here.