Ive been using AdSense for a very long time, and I think Ive got it figured out pretty well. I understand how ads need to look to get people to click, I know where to put them and its pretty rare for my units to show anything other than ads that are highly contextualized. Maybe I could optimize some of the pages a little better (heck, no ones perfect) but on the whole, I think I can say that I know how the system works and Ive got it working pretty well for me.

And yet, there are times when I havent a clue whats going on.

I can look at a week of stats and see my CTR bumping along quite nicely, then all of a sudden there will be a dip one day and a spike the next.

Nothing will have changed. My set up will be exactly the same. The ads will be the same and my traffic sources will have been the same.

So why fewer people will have decided to click one day and more the next is beyond me.

Of course, I dont just see these strange occurrences on my AdSense stats. This pattern can be repeated in my own advertising and in my server logs.

Sometimes, the reason can be fairly clear. I expect traffic levels drop around Christmas and other holidays. Weekend traffic is different to weekday traffic. Even a big news story can drive people away from my site towards other sites... I guess.

But usually, Ive got no explanation at all. Of course, as long as drops dont last too long, I dont care -- especially if my overall earnings remain the same. (Although if a drop lasts more than a couple of days then Im going to start looking very closely at the ads Im being served, at the page the ads are on and at the source of the traffic Im getting. If theres no bounce-back by the end of the week, Im going to be making changes.)

Its pretty rare that my stats go off the rails for more than a day though and Ive come to regard these ups and downs as just part of the mysteries of the Internet. Its a constant reminder that there are some things we just cant control.

Its very annoying though.