You’ve read all of your free items this month. To register, get added benefits and unlimited access to articles, Become a Member. Already a Member? Sign in. Advertising Age Article Abstract More Nielsen Problems: Under-Counting 'Unique Visitors' by 5% Company Making Progress in Fixing Initial Glitches, but Finding More By Michael Learmonth, Published on November 17, 2010 2 Want more Access, Content & Connections? Upgrade to Membership. We are glad you are enjoying Advertising Age. To get uninterrupted access and additional benefits, become a member today. Register Now Already a member? Log in.