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AdSense: How does Google measure 'impressions'?

My internal stats contrast starkly with Google's!

7:54 am on Jun 23, 2003 (gmt 0)

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I use wusage (a popular and generally well-respected stats program) on my site to measure traffic by section.

My blog -- which now has AdSense ads on it -- has been getting around 1500-2500 hits per day on average lately.

AdSense, however, has been showing 35-50% that number of impressions!

So that raises some questions:

- What does Google count as an impression? Perhaps it has a database of known spiders, and is specifically excluding those hits from its stats?

- If so, are 50-65% of my hits really coming from spiders?!?

What other explanations could their be? I'm not alarmed, really, but definitely confused about this situation.

Has anyone else compared Google's stated impressions with your own measurement of impressions (via WebTrends, wusage, etc.)?


8:20 am on June 23, 2003 (gmt 0)

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around 30% of my traffic comes from spiders of one kind or another, though I include in that category any kind of automated retrieval, ie clients pre-fetching lots of pages. I'm guessing that none of those will actually run Javascript, and hence won't show up as impressions on Google.
5:43 pm on June 23, 2003 (gmt 0)

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I just did a quick comparison of yesterday's AdSense impressions with my logfile analyzer's pageview report, and the numbers are similar.

The logfile analyzer shows slightly more page views than impressions, but that's because I don't have AdSense on all of my pages.

(I use only plain-vanilla HTML files and links, FWIW.)

3:40 am on June 24, 2003 (gmt 0)

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Well, I'm a bit embarrassed. I usually tease other folks about not RTFM, and here, in black and white, is what I overlooked from Google's AdSense FAQ:

Why do I detect a different number of impressions than what shows in my reports?
We count impressions only when our ad code is executed by a user's browser. There are therefore several factors that would cause a discrepancy between the impressions you register and the impressions listed in your AdSense account. Instances that wouldn't execute the code include:

  • Other spiders, robots, crawlers from other search engines
  • Browsers that don't have JavaScript enabled or don't support JavaScript
  • Browsers that don't support the <iframe> tag
  • Programs that people may have written to grab website content

Oops :D

9:20 am on June 24, 2003 (gmt 0)

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Okay, I've done some actual hands-on testing here, and I'm once again back to being puzzled.

I clicked on 41 pages (that display AdSense boxes) throughout my site earlier this evening, waited about ten minutes, and... the AdSense reporting page shows NO new impressions.

Adding to my concern is the fact that I show AT LEAST 2,017 hits yesterday to pages on my site with AdWords boxes, and AdSense shows 559 impressions.

When I last checked (via automated means), around 94% of my visitors were using iframe-and javascript-capable browsers. Just to be conservative, let's say 90%.

90% of 2017 is 1815. That leaves 1256 impressions unaccounted for. I can't imagine that well over HALF the hits on my AdSense'd pages are coming from spiders!

I'd sure like to test this out by clicking on a few ads, but for obvious reasons, I don't feel comfortable doing that. :O

Anyway, does anyone have any additional insight into:
1) what may be causing this large discrepancy
2) what other tests, if any, I should do
3) whether this is anything I should even spend time worrying about (after all, we're being paid by clicks, not impressions; I just hope all my clicks are being counted! :)


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