Google Analytics is a great piece of software, but it has a major flaw in the way it tracks average time on site and bounce rate.
Let’s take the worst-case scenario as an example; someone visits your site, spends five minutes reading an article, but has no further interaction with your site and then leaves. Google Analytics measures the time he spent on the site as the difference between when he entered and his last page view. In this scenario Google will see the difference as zero and call it a bounce when it was actually an engaged visitor.
Here is a great article on the subject that explains it better than I can, click here to read it.
I changed the code to be five seconds instead of ten for my own site because I wanted time on site to be accurate within five seconds. The average time on site for this blog went from 47 seconds to 3 minutes and 21 seconds and the bounce rate dropped dramatically.
Using this trick to make Google Analytics more accurate is especially important if you are charging advertisers for banner space. Your media kit will be grossly under-reporting the time people are spending engaged with your content if it contains statistics from Google Analytics and you haven’t added this code to your site. I’m surprised Google doesn’t include this by default.