This is a summary of what Google posted on their AdSense Blog.

First things first: eCPM is the metric of the expected revenue from every 1,000 page impressions. Of course, the higher the number, the better. If you run AdSense across multiple sites and industries, you will see this number vary widely. I always recommend spending your time doing SEO on the pages with strong eCPM.

Okay, now let’s get into the fluctuating revenue numbers …

1) Look at the page impressions. Simple enough, but sometimes overlooked. Your revenue could be bouncing because your traffic levels are bouncing. Check your analytics for clues as to why your traffic is bouncing.