The web user is the most watched consumer ever. For tracking purposes, every large site drops literally dozens of cookies in the visitor's browser. In the most comprehensive investigation on the matter, The Wall Street Journal found that each of the 50 largest websites in the United Sates, weighing 40% of the US page views, installed an average of 64 files on a user device. (See the WSJ's What They Know series and a Monday Note about tracking issues.) As for server logs, they record every page sent to the user and they tell with great accuracy which parts of a page collect most of the reader's attention.
But when it comes to measuring a digital viewer's commercial value, sites rely on old-fashioned panels, that is limited user population samples. Why?