The comScore report is indeed dubious, and there is a great deal of debate going on between supports of "panel-based" traffic metrics (people who seem anxious for the comScore IPO), and people who know what they're talking about. :P
Below are the details that should provide plenty of fuel for discussion... but, for those not inclined to spend time with the details, here's the short story...
- Only 8.9% of the weekly new users either deleted or refused cookies
- Google Analytics was only 1.5% high in their report of absolute uniques
- As I suspected, the number of users who manage cookies is dwindling
Unique User Test
We used the following criteria, in this order, to determine that we had a new, unique user.
1) New visitor is not a site member
.......(logged in members are a known entity)
2) New visitor with no previous cookies from our domain
.......(either never visited, or deleted cookies)
3) The visitor came in via some page other than the home page
.......(regular users tend to bookmark the home page)
4) The visitor came in with a referral from another domain
.......(regular users tend not enter the site from an off-site link)
5) The visitor had a "User Agent" identifier
.......(some bots now have blank user agents)
6) The visitor's User Agent was not a known spidering bot
.......(we compared against a list of 50 known bots)
7) The visitor views pages at "human speed"
.......(rapid-fire visits are likely unknown bots)
8) The visitor's combination of IP, OS, and User Agent is unique
9) If the user logs-in (a member), they are not counted
If the above was met, we logged the user and wrote a new cookie with a one year expiration date.
So far, in 168 hours (one week), we tracked the following statistics:
Unique users recorded ---> 121,718
Combined users deleting & refusing cookies ---> 10,815 (8.9%)
Number of repeat users deleting cookies ---> 7,439 (6.1%)
Number of repeat users refusing cookies ---> 1,186 (0.9%)
Number of one-shot visitors refusing cookies ---> 2,190 (1.8%)
Number of members visiting for the week ---> 3,081 (124,799 uniques)
Google Analytics "Absolute Unique First Time Visitors:" ---> 123,498 (off by 1.5%)
.......(for the same time period as the user/cookie test)
We're going to continue running this, and compare 30, 60, and 90 day data... I suspect the gap between our numbers and Google Analytics will widen and see a slight increase in cookie deletion.