You'll want to make sure you have extensively detailed timelines, then, or else your data won't have much use. Correlation vs. causation, basically. For myself, for instance, I used to read a minimum of two novels a week when I was in high school. Now that I'm actually working and supporting myself and my spouse, I get through maybe a novel a month. I didn't previously read anything online; now I read a dozen or so short story e-mags and participate in writing workshops. I found these podcasts specifically because I was looking for a way to "read" more without having to spend more time actually sitting and only reading. I imagine a lot of people who are hardcore audio fans have similar situations; long commutes, boring jobs, whatever.
So my chart of "amount read" would start out very high and deteriorate steadily over the past ten years or so, with EP, PP, and PC having no real impact on it whatsoever. It's not a case of actual reading crowding out audio or audio crowding out reading; it's a matter of available time and portability.
