It is believed that CTR depends on a number of factors like ad placements, amount of traffic, traffic sources etc with the emphasis given on ad placement. But there is another deciding factor that nobody mentions and that is how attractive these ads are. I have been running Adbrite for sometime but because of the irrelevant ads, the CTR is almost non existent (520K impression and 6 clicks!!). So I have pushed the ads to the bottom of my site and just earning from the CPM ads. Recently, I was going through the advertisers list and when I tried to sort it according to the no of clicks received I was shocked to see that I had received hundreds of clicks, but only on CPM ads. So they didn't count. Some of these ads had a very high CTR, and they all came from the zone that was lying way down the bottom of the page, to the side - the place I believed nobody would look. I was so wrong. If the ad is good looking (preferably girls, for male audiences ), it will be clicked no matter where you place it. See some examples: CTR - 0.72% CTR - 1.9% CTR - 2.8% Even this got CTR of 1.08% LOL!
Agree the ad media has a huge impact on CTR... it also depends on the audience I mean you won't sell ice to an eskimo but you might sell him a heater. I guess it's all about relevance.