_setVar() and the zero bounce rate bug in Google Analytics [StartupCTO]

Last Updated: 20 Oct 2023

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Author: 135.181.114.235

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= _setVar() and the zero bounce rate bug in Google Analytics =

** UPDATE - Jan 27, 2009: Google has reportedly fixed the bug in GA that was causing the problem described on this page. Thus the fix is no longer necessary, but I will leave it here for historical reference. **

Google Analytics has a ''_setVar()'' function that lets you assign a single user-defined variable to be passed to your reports, which you can then see (and filter on). 

Unfortunately, using ''_setVar()'' on each page seems to break the bounce rate calculation in GA. This is because ''_setVar()'' apparently records a second pageview when it is called, meaning a visitor who left after their first pageview didn't actually 'leave' in the eyes of GA because they had a second page view secretly recorded via ''_setVar()''. Thus the bounce rate in Google Analytics drops to zero (or near zero) because //every single visitor// is getting hit with two pageviews. 

Conveniently, the value that you assign to ''_setVar()'' is stored by GA in the ''_utmv'' cookie, and the tracking code actually reads this cookie and passes back the value to Google Analytics, which records it for //every single pageview//, regardless of whether you set it with ''_setVar()'' on that particular pageview. Therefore **you don't need to call ''_setVar()'' unless you actually want to //change// the user defined value**. In fact, you //shouldn't// call it any more often than you have to, because you don't want to be creating these extra pageviews. 

This lends itself to a nice solution to the problem: we'll simply check if the value we want to call ''_setVar()'' with is already in the ''_utmv'' cookie. If it is, we'll skip calling ''_setVar()''.

<code javascript>
/*
 * getCookie(): given a cookie name, get its value. Returns null if the cookie can't be found.
 * From http://www.webreference.com/js/column8/functions.html
 *
 */
function getCookie(name) {
  var dc = document.cookie;
  var prefix = name + "=";
  var begin = dc.indexOf("; " + prefix);
  if (begin == -1) {
    begin = dc.indexOf(prefix);
    if (begin != 0) return null;
  } else
    begin += 2;
  var end = document.cookie.indexOf(";", begin);
  if (end == -1)
    end = dc.length;
  return unescape(dc.substring(begin + prefix.length, end));
}

/*
 * utmvCookieCheck(): given a value, read the __utmv cookie and see if
 * that value is already set. Return true if so, false otherwise.
 *
 */
function utmvCookieCheck(value) {
	var utmvCookie = getCookie("__utmv"); 
	
	if (utmvCookie == null)
		return false;
	
	// get rid of the Google's domain prefix ID, which appear on all
	// GA cookies
	utmvCookie = utmvCookie.replace(/^\d*\./, '');

	return (utmvCookie == value) ? true : false;
}

</code>

Now, just wrap your call to ''_setVar()'' with a call to check if the cookie exists first:

<code javascript>
<script type="text/javascript">
var gaJsHost = (("https:" == document.location.protocol) ? "https://ssl." : "http://www.");
document.write(unescape("%3Cscript src='" + gaJsHost + "google-analytics.com/ga.js' type='text/javascript'%3E%3C/script%3E"));
</script>
<script type="text/javascript">
var pageTracker = _gat._getTracker("UA-XXXXXX-X");
var userDefinedValue = 'CHANGE_ME_TO_YOUR_DESIRED_VALUE_FOR_SETVAR';
if(!utmvCookieCheck(userDefinedValue)) {
	pageTracker._setVar(userDefinedValue);
}
pageTracker._trackPageview();
</script>
</code>

That's it. Now ''_setVar()'' will only be called if the value isn't already set.

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