That's a tricky subject, especially when dealing with channels - which now show the ctr of ad impressions rather than a ctr per page like an aggregate report does. I think a normal ctr is around 6 percent, but I could be wrong. That number goes down for a forum or any other type of site where users become ad-blind from frequent use. When you are looking at your channel stats, just calculate your ctr directly for page views based on the number of ads you are showing per page. It's pretty easy, just multiply your ctr by 2 or 3, depending on what you are showing. I think a really good rate is 10 percent, and I just saw someones stats today that were around 30 percent, so it just depends on the kind of site you have. There are a lot of people with CTRs under 2 percent and making good money just because of their traffic numbers or because they have sites in the right niches.
I don`t think you need to care about CTR until you are are going to click your ads, or resort to some automatic bots clicking them. Its not in your hands to control CTR. But on a serious note, forums CTR can go as low as 1 to 2% while good design sites can boasts upto 15-20% CTR or even above on some special days.
Overall average of CTR will be fluctuating between 5%-10% . All though forums tend to have low ctr's it is possible to get the same results as the average. Nandini, what happened to your webatlas.org directory? Did you get banned?