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Quantcast claims that SO receives 1.8 mm unique visitors per month, and 17.2 pageviews per month:

You can't see this part, but Google Analytics shows:

3,768,298 Absolute Unique Visitors
18,180,413 Pageviews

for the last month.

Pageviews seem to match up very closely. Why would there be such a huge difference in unique visitor counts?

(It makes a big deal in the ranking scheme for Quantcast.)

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Although you're referring to the traffic data of Stack Overflow, I don't think this constitutes as a meta question, as the question is really about discrepancies between GA and Quantcast. This might be better off on ServerFault. – Kyle Cronin Jun 29 '09 at 16:59
or superuser.... – jjnguy Jun 29 '09 at 17:04
I think it's meta enough to be here, since it is very site specific. – Jeff Atwood Jul 1 '09 at 5:58
up vote 4 down vote accepted

IIRC Quantcast estimates the unique visitors from the users that are carrying their cookie information.

Uniques are a standard measure of audience size available from sophisticated analysis tools including Quantcast. Although many tools label them "visitors," uniques technically count the distinct cookies received from or sent to visitors.

Quantcast is the only measurement service to provide audience data for both unique cookies counts and people. The Quantcast inference-based direct measurement model takes into account numerous factors including the frequency of visitation and the respective balance between work and home access to build a translation of cookies to people that is unique to each digital media property.

The model was introduced in June 2008 and is detailed in our white paper, 'Cookie Corrected Audience Data' available to view or download here.

More info here.

As a courtesy, Quantcast provides 'rough' audience estimates for non-Quantified digital media properties. These rough estimates are just that - far from perfect. We strongly recommend non-Quantified publishers refine the accuracy of our reporting on their audience activity by joining the Quantcast Publisher program at absolutely no cost.

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I read the paragraphs you quoted but I don't get it. :\ – Michael Pryor Jun 29 '09 at 18:13
It's something to do with their secret sauce. :) – Ólafur Waage Jun 30 '09 at 21:06
Also, Quantcast is flagged as an "Advertising" tracker with Ghostery which a lot of SO overflow users would be running and thus would likely have the entire "Advertising" group blocked. – Michael P Feb 1 '14 at 5:56

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