The eCPM metric provides an estimate of how much revenue you can expect to earn for every 1000 page impressions. For example, if you serve 10,000 page impressions and earn $40, your eCPM is $4. If page impressions increase to 30,000, you can predict that you'll earn $120 given the $4 eCPM.
If your AdSense page impressions have declined, you should determine if traffic to your entire site is declining as well. A web analytics tool such as Google Analytics can provide you with this information. In addition, you should check your pages for unpaid public service ads (PSAs).
If your eCPM is down, you'll need to dig one level deeper and find out if your contextual or placement targeted ad performance has dropped. You can also find this data in the Advanced Reports tab using the options shown below.
For placement-targeted ads, you should analyze how much total placement-targeted revenue you are receiving and the average eCPM. Changes in either of these metrics usually indicate that advertisers are beginning or ending campaigns targeted to your site. Again, placement-targeted campaigns are more likely to be short-term than contextual campaigns.
That's all we have time for today -- now that you have a better understanding of what factors can affect revenue, don't forget to check back later this week for the second part of this series. We'll be discussing ways to treat revenue fluctuations based on the symptoms you've discovered.