A WebmasterWorld thread has complaints from some Google AdSense publishers of low click through rates (CTR) compared to the average.

I don’t use AdSense that much, so my data looks fine but there are a few people complaining about a recent huge decline in CTR on their ads. One said their CTR dropped as much as 40%.

One publisher said it is normal for December:

Have you noticed a low CTR recently?

Forum discussion at WebmasterWorld.