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CPM = paying to display an ad 1000 times
CPC = paying for getting a click on an ad
..... I cannot help but notice that irock does not volunteer a figure for their own CPM. ;)
Personally I find CPM a very useful figure as it is mathematically a function of CTR and CPC. Often one of those goes up, but the other goes down.
I use Excel to give me a daily figure for CPM based on a 7 day moving average, which smooths out the obvious weekly cycle to AdSense (for tourism sites anyway.)
The number of impressions you put out is (broadly) within your control, rather than AdSense's control. So I have a separate daily 7 day moving average for impressions.
The two graphs give a good indication of whether you have a problem, and if so whare.
On a personal level, I have not seen any fulfilment of the prophesies of doom that are sometimes expressed on this board!
3 sites, together producing around x impressions have y CPM. I now added a new site, more focused and in a high cost ad industry. This site changed my over all impressions to 1.25x, so has only a quarter of the impressions of the other sites combined, but incresed my CPM to 2.5y.
Morale is that topic to topic and site type to site type the potential for AdSense varies greatly, surely by an order of magnitude in the extreme ends.