I think Google Ad Planner is pretty amazing and chock full of information. But to some webmasters, the information they have is either so accurate it is scary or totally off and worthless.
In a WebmasterWorld thread, Google AdSense representative, AdSenseAdvisor, replied to questions about where they source the data, how to opt out and what to do if the data is off. Let me quote what Google said:
To estimate website traffic we use a combination of two approaches: 1.) We extrapolate website traffic from sample data we collect from a variety of sources. For our sample data Google Ad Planner combines information from sources such as aggregated Google search data, opt-in anonymous Google Analytics data, opt-in external consumer panel data, and other third-party market research. The data is aggregated over millions of users and powered by computer algorithms; it doesn't contain personally-identifiable information.
2.) Individual websites can opt-in their Google Analytics data to improve the accuracy of data displayed about their site in Ad Planner.
This methodology is similar to that of other 3rd party online measurement tools, which also use hybrid approaches that blend server side measured numbers with sample/panel based estimates.
Google Analytics Opt-in Data @Cancellara - Did you opt-in your Analytics data into Ad Planner? We don't use your Analytics data in Ad Planner unless you explicitly choose to share it. So unless you've chosen to opt-in your Google Analytics data to Ad Planner, it's possible that the Ad Planner estimates are different than your Analytics statistics.
@Oxydada - When looking at your Analytics reports compared to Ad Planner, are you comparing US to US figures? Ad Planner by default shows US numbers while Analytics shows worldwide figures, so this could be the cause of the discrepancy.
@rashidjaved11 - Please file a ticket with our support teams and we'll try to help diagnose your issue. http://www.google.com/support/adplanner/bin/request.py
@aish1108 - If you've checked everything and you're sure your tags are on all your pages, then you should opt-in your data. The "low tag coverage" message is just a warning to tell people to check their tag coverage. It's possible that our check is making an error since we're basing it on Google crawl data and trying to detect your Analytics tags from the crawl. The data you see in your Analytics reports is what we'll display in Ad Planner
Forum discussion at WebmasterWorld.