To get an accurate number of the duration of a visit you'd have to make a ajax request every second or so, or somehow make a request whenever the user closes the tab. Also, you need to measure if the tab loses focus. To get even more accurate you could store the amount of scroll the user has done over the duration, to see if the visitor actually read to the bottom. These ways are seen as quite invasive just to get the duration of a visit. I know that e.g. woopra makes a request every 10 seconds with different metrics, so they would at least get better accuracy than google analytics.
Understood. Other services do accurately measure this however. ChartBeat[1] tries to differentiate between users who are reading, writing and so on. Re: your example, it does literally keep track of how far you scrolled as well.
Unfortunately, my personal website is not their target market. Plans start at $9.95 per month: infinitely more than my hosting cost and also overkill for whenever I'm not front paged. It'd only really make sense for me if it had pricing based on usage, which I can almost guarantee they'll never launch.
I'd argue that Google Analytics is vague in defining or explaining their terms. In a perfect world, yes, they'd offer "pings" to see if a user is active, but even without that I'd at least hope they clarify their terms.
Misunderstood terms lead to misunderstood statistics.
The less misunderstood statistics, the better.
