Year to date, and basically the last year+, interest based clicks resulted in the best RPM for me. Jan/Feb 2014 interest based RPM was 40% better than contextual RPM. Starting just about a week ago, that's totally changed. In the last 7 days, now contextual RPM is 80% better. That's quite a dramatic change. Not good for me. It's not been CTR really; that's off a tiny amount. But the CPC for interest based clicks is only 1/2 what the average of the last 2 months is. Concerning for me. You might want to check your stats.