The New York Times article How Many Site Hits? Depends Who's Counting from 2007 by Louise Story looks at the accuracy of website visitor tracking for online publications of Vogue and W magazines (Style.com). The problem is anomalies between internal monthly visitor counts e.g. September 2007 by publisher Condé Nast (1.8 million) compared to those of external agencies such as ComScore (421,000) and Neilsen/Net Ratings (497,000) whose counts have so much influence on advertisers and advertising rates.
As a Google Analytics Certified Partner we use Google Analytics to report and analyze website visitor tracking data directly from the websites of a wide range of organizations to allow them to measure results for lead generation, online and offline marketing campaigns, online and offline advertising, membership campaigns, online ordering and online sales.