Contextual Ads
AdSense Smart Pricing, One Bad Placement Can Devalue All Clicks
JenSense has an interesting piece on the inside workings of Google AdSense's Smart Pricing, which rewards publishers with higher or lower payouts based on quality of conversions.
One of the most notable revelations gleaned from an AdSense support member is that:
Smart pricing affects an entire account. It is not on a per page or per site basis.
Which means you should be careful about where you stick your AdSense code, it could be counter productive and actually lose you money across your entire account if placed anywhere it performs poorly in the eyes of Smart Pricing.