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There is "Clickstream" with upstream and downstream sites.
There is at the downstream sites Doubleclick.net
The percentage listed in Alexa is very close to my CTR at AdSense
I tried to test several other sites.
The online version of a big German newspaper - bild.de
showed the main adserver of this newspaper
So if there are only 10 internet users and 1 alexa bar user then 10% of the net is running the bar, so they know how to multiply their toolbar data to get a fairly acurate estimate of total usage.
Very random and small numbers in my example I know :)
1. Traffic Rank a million miles away from directly measured Quantcast
2. Sites linking in approx 1.4% of actual real world.
3. "upstream or downstream" is a total nonsense. Downstream in particular includes XYZ affiliate programme with an astonishing 17.65%. A subsidiary company of Amazon.com can't determine that I send 5 - 10 X that affiliate traffic to Amazon compared to XYZ?
No mention of AdSense anywhere.
As I said:
Alexa in my experience is useless and so far wide of the mark to be laughable
BTW, Alexa counts visits while AdSense counts page impressions.
So Alexa's 'CTR' * pageviews/visitor should match the AdSense CTR.
That's assuming you have adsense on all pages.
My site only has about 20% of it's pages with adsense on.
Anyway, I have never bothered with Alexa's data. Take it with a huge pinch of salt.
Which is why I finished with..."I have never bothered with Alexa's data. Take it with a huge pinch of salt."
I had always much fun with Alexa data.
Last time, somebody claimed he will have next year 50.000 EUR pro month from advertising at his internet project.
His Alexa data showe about 5% of my current traffic.
So he wants a 500 times higher eCPM, because my 20 times higher traffic earns only little bit above 2000.-EUR a month.
Great was a request for SEO work.
I found that they claim huge visitor numbers at the contact info.
I answered the estimation for SEO work
"After a half year of hard work, we will reach the visitor numbers, You are claiming just right now on Your site"
I did not get the order, but the vistors numbers vanished from their site :)