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.
<script type="text/javascript">
var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-XXXXXXX-1']);
_gaq.push(['_trackPageview']);
setTimeout("_gaq.push(['_trackEvent', '25_seconds', 'read'])",25000);
(function() {
var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
})();
</script>
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.