I have both installed on my site on every page. I literally have them both in the same file so they cannot be included separately one without the other. Quantcast consistently shows dramatically lower numbers than does Google Analytics. I don't get this at all. For instance, one day last week saw 8,086 visits according to GA. The same day saw 1,368 according to QC. GA thinks my traffic is more than five times what QC says. What gives? Also, I have Adsense ads running on my site. The Adsense stats agree pretty closely with Analytics. My CTR is roughly 2%. If I were to believe QC it would be more like 10%. Which is accurate? Why?
I have this issue as well. First, make sure that you are looking at global traffic in Quantcast, as they show US only by default. But the issue is deeper than this. For me right now, pageviews are only about 20% lower in Quantcast, but visits and visitors are 50% lower. I do have one explanation which explains some of the difference. I have two sites in a network, and the visitors for the network are higher than for both sites combined. It tells me that 29% of people are from syndicators, and almost all of the syndicators are Google domains. When someone clicks on a result from my site in Google Images and my page is loaded within an iframe from Google, I believe that Quantcast does not count this as a visit to your site (or a pageview or a visitor), and only lists it under syndicators. Google Analytics on the other hand does count this. One way to get more accurate stats from Quantcast is to set up a network with just your site in it so that syndicators will be counted. Examiner.com has done this, and if you look at their Quantcast page you can see 5% of their visitors are only counted through sydicators. In addition to Google, they also have a large amount of syndicator visits from StumbleUpon which also renders the page within an iframe.
This is pretty strange. I set up a network with my site as the only member in my Quantcast account. The network reports 160K monthly visits vs. 21K for the site itself. The 160K is still about 60K under what GA reports, but I'm more concerned about the 160K vs. 21K. Does this mean that Google and others are framing my site? Or am I completely confused?
Google web search does not frame your site but Google images does. Those results might make sense if a significant portion of your traffic is coming from Google images (if you are unsure how much traffic you are getting from Google images, there are some customizations that you can do in Analytics to segment that traffic, by default Google images is not separated from Google wen search). Quantcast will tell you who the syndicators are if it is not just Google.
The overwhelming majority of syndicators for my site are various Google flavors, but I see nothing about Google image search. I don't get it.