A fantastic post by Francois Derbaix, CTO at top French travel site Toprural.com, tells us that Google is as susceptible as anyone else to imposing conditions that make its services look good when you analyse your site traffic.

Comparing the stats for Toprural delivered by Google Analytics
with his own 3rd-party solution, he finds that, while his own system (AT Internet’s
XiTi) says 37.8% of visitors come via Google, GA says it’s 71.8%. The
core of the problem, he discovers, is the good old cookie window. It
turns out the default cookie window Google ascribes to visitors that
arrive on a site via Google is six months. Six months!

So, every visit a user happens to make to a site for the six months after having once visited that site via Google is ascribed to the search engine. The standard window for display media is 30 days, a sixth of the length Google has opted for, while we’re typically asked to assign search cookie windows of a couple of days at most.

Monsieur Derbaix gives an example of how the cookie window might impact your site stats (the extra joy of this quote is a demonstration of how Babelfish has translated his post from French to English): Example: Pierrot visits your site 2 times in a direct way (it types your URL in the bar of address of the navigator), after which it comes there once through the results from a research on Google. Then he visits the site 3 times still on line, without passing by again by Google, in the 6 months which follow.

“How much visits since Google, GA will indicate? 4 (1 + the 3 following ones). How much direct visits indicated? 2 (two first). Whereas actually there is 1 well visit by the means of Google and 5 natural/direct visits.”

In other words, in his example, a user that actually visited the site via Google just once, is credited to the search engine four times.

It’s crucial to point out that the six-month window is just default and configurable inside GA, but it’s safe to assume that few GA users will a. be aware/understand the importance of this setting or b. be able/willing to do anything about it.

For me, this is an indicator of lots of things: first, perhaps more explanation of why Google launched Google Analytics for free in the first place; second, the real importance for site owners to understand the detail of how online advertising works; in a detail like cookie windows, for example, lies crucial information; and, third, the importance of independent providers in helping agencies and advertisers make sense of the wealth of info now at their disposal.

If you make decisions based on data, then that data has to be reliable.