Google Analytics is a free, very robust web analytics tool that I’d recommend for implementations both large and small.  In my opinion, though, one of GA’s greatest strengths also serves as one of its greatest weaknesses: it’s so easy to use.  A slick, AJAX-y interface can have the dangerous effect of sending our brains into autopilot, which in turn leads to lazy interpretation and analysis.  In order to truly understand the metrics, we have to see past the pretty graphs and ask ourselves questions like, how was this data collected? and what equation defines this metric? Today let’s talk about a metric that is often insufficiently understood: referring source.

Referring source is one of the most useful reports in Google Analytics.  This single report gives us insight into both quantity and quality–how many visits and what types of behavior a traffic source drives.  Here’s an example of the referring source report from Searchability’s Google Analytics account:

At first glance, this report seems dummy-proof.  But here’s where understanding of this report typically breaks down.  See if you can answer this question with an unwavering degree of confidence: does referring source persist across multiple sessions, or is it reset upon each visit?

This is a question that deserves investigation, as the answer carries weighty implications.  Consider a scenario in which a person visits your site via a banner ad, leaves without making a purchase, and then reaches your site via a search engine two days later, this time making a purchase.  To which referring source will Google Analytics attribute the purchase, banner advertising or search?

As it turns out, referring source persists across sessions.  Below is a statement from Google’s documentation on Google Analytics and cookies:

When visitors reach your site via a search engine result, a direct link, or an ad that links to your page, Google Analytics stores the type of referral information in a cookie. The parameters in the cookie value string are parsed and sent in the GIF Request (in the utmcc variable). The expiration date for the cookie is set as 6 months into the future. This cookie gets updated with each subsequent page view to your site; thus it is used to determine visitor navigation within your site.

To reiterate from the citation, the cookie containing referral information persists for six months.  So, unless your visitors delete their cookies, their initial referring source will persist across any subsequent visit within a six month period.

How will this affect how you do web analytics?  If holistic referral attribution analysis is critical to your business, you will need to consider a method of capturing a visitor’s most recent referring source.  One possible solution is capturing the HTTP_REFERER in a session variable.

I first began questioning the nature of referral persistence when I was managing online media and analytics for a small website.  We had stopped running a particular banner advertisement, and yet we were still seeing several visits a day from that banner.  The explanation, of course, is that these were returning visitors who had initially been referred by the banner.

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