purplecape - 3:26 am on Oct 15, 2009 (gmt 0)

The only problem is no one knows what percentage of the ads being shown on a given site are interest-based, or even if a significant percentage of all ads are interest-based. Does anyone have any solid data on this? As far as I know, it doesn't exist.

I have an alternative explanation, that I think works without resorting to interest-based advertising. Basically, I saw something similar happen on my site, long before IBA came along. The site is a fairly small niche, but years ago ads tended to cluster around 3 or 4 related but distinguishable topics. One of those clearly paid significantly more than the others per click. Over the course of a couple of years, those better-paying ads began to appear throughout my entire site. CTR went down because they weren't as relevant in other areas, but they were relevant enough that they did get clicks. Overall income didn't change much, but this was in a time when many site owners were seeing their income go down, so the effect, arguably, was positive.

Something similar may have happened to dataguy.