• by Joe Mandese  @mp_joemandese, November 3, 2009

If I eyeballed the chart Robert Coolbrith just flashed quickly at OMMA Ad Nets this morning, it looks like “Class II” now represents something like 15% to 20% of Yahoo’s display advertising business.

Coolbrith is a vice president of equity research at ThinkEquity, and the so-called class II  inventory is the kind that is sold for less than a premium via ad networks, and exchanges like Right Media, which also happens to be owned by Yahoo.

Coolbrith did say that the estimates were old and need to be updated, but that’s still a remarkable share of Yahoo’s business, which may be a good thing or a bad thing depending on how you look at it. It’s a bad thing, obviously, if the portal is getting lower CPMs for the class II inventory, which it presumably is. But it’s a good thing, because it’s helping to offset a decline in Yahoo’s premium display advertising sales.

And I’m not saying which is the chicken and which is the egg in this trend, but the reality is that the trend mirrors what’s been going on in the greater online display advertising marketplace. According to some benchmark research by the Interactive Advertising Bureau, Coolbrith said that 70% of the inventory sold today is class II and only 30% is class I inventory. Of course, publishers are generating the predominant amount of their revenues from the relatively smaller share of class I inventory.