Http is stateless
As PageView is easy to measure, a Page Time is of a different magnitude.
A Web server knows when the page is being sent, but the Web server can’t know, if the visitor is still on the page or already on another of a different Website. If the visitor stays on the same Website, the Web server knows the time on a page, when he is requested to sent the next page. If the visitor goes away to another site, the Web server doesn’t know at all.Due to the fact “http” is stateless, Page Time is a bit harder to measure than Page View.
Most web analytics packages do estimate time spent, this can be wrong from 4 seconds to 10 minutes.
Still, they can only make an estimation, as they don’t know, when you leave the site.
The Appmetrix technology solves the 'stateless'
The Appmetrix technology of Engago Technologies has solved this problem.There for the exact time spent on a website or webpage can be measured.
PageTime & Unique Visitors the New Metrics for Advertising
PageTime uses the AppMetrix developed by Engago Technologies Ltd.