Bounce Rate is the percentage of visitors who just access one page in a website and then leaves. If you have a popular blog, regular visitors will not access more than one page in a visit. In such cases, Bounce rate based on page view would be meaningless. Third Party Web Analytics Software like Clicky Web Analytics had been using time on page as a factor for determining Bounce Rate. According to Clicky, any visit that lasts more than 30 seconds, even if it is a single page view, is an engaged visitor.
Google Analytics team has taken a step further by allowing webmasters to set the minimum time for Bounce Rate by modifying the Google Analytics code.
var _gaq = _gaq || ;
setTimeout("_gaq.push(['_trackEvent', '25_seconds', 'read'])",25000);
var s = document.getElementsByTagName('script'); s.parentNode.insertBefore(ga, s);
As you can see, the setTimeout function sets the delay in tracking event. In this case, 25 seconds is the minimum time set for calculating a bounced visit.