coachm - 10:27 pm on Jun 16, 2008 (gmt 0)

Impressions and eCPM both turned south on that day over 10 major channels I track. (Each channel represents a different niche; all the niches fit under a healthcare theme.) Since imps are pretty much a factor of traffic, I am figuring the Goog serp updates that are being discussed in the Search forum have given me a downward bump. But why would eCPM change also? That's got me scratching my head.

Well, you could call it a glitch, and pretend you've explained something OR....

...with serp updates it's possible that your traffic volume may remain the same, but that the tendency for the "new traffic" will be to click less, which could affect your ecpm. In other words, the characteristics of your visitors changed.

Or, google takes into account how the visitor/clicker got to your site (ie. what keyword phrase) and factors that into your final click payment. SO, as your serps change, so does your ecpm.

Both are plausible, even likely possibilities. Neither is provable.