As the internet continues to overtake television as the dominant entertainment medium of the era, technology to measure online viewership is becoming ever more sophisticated. “With TV, you know who went in your front door, what they bought and how long they stayed. On the Web, tools that monitor your site can tell you not only who visited and what they bought, but which aisles they visited, what they touched there, how long they lingered, where they came from before the store and where they went after.”