The Web Analytics Association (Jason Burby and co.) recently releases
26 Standard Definitions to Promote Consistency among us Vendors (PDF) - this is GREAT news and should make it much more straightforward to communicate web metrics among not only traditional report consumers, but also more experienced analysts! However we are not there yet ...and please see this post as an inspiration to continue the splendid work. One of the standard definitions are “
Visit Duration” – specifically described as follows:
WAA Standard definition: Visit Duration Definition/Calculation The length of time in a session. Calculation is typically the timestamp of the last activity in the session minus the timestamp of the first activity of the session.
Comments When there is only one piece of activity in a session (a single-page visit or singleevent visit), no visit duration is typically reported.
This is a fine definition and one that I certainly agree on. BUT then comes questions on the subsequent metric that we could call “
Visit Duration Average” – first question is what to name the metric and second question is on how to calculate the metric. Before concluding anything. Let us take a look at Google Analytics and
IndexTools (because that is funny enough what I use here on my blog – still waiting for those Omniture blokes to call me ;-)
- Avg. Time per Visit 4m 46s (IndexTools)
- 00:07:09 Avg. Time on Site (Google Analytics)

Two different naming conventions and two different calculations (assuming we both collect and accurately calculate the metric). *send me your definition from Omniture, Visual Sciences or other decent Web Analytics vendor and we might be able to help out our good friend Jason. I searched through my WebTrends Marketing Lab account and could not find anything. Anybody?
So first (and I am completely biased here) – I do not really think the Google Analytics “
Avg. Time on Site” naming convention is clear enough confirming that we talk about a Visit (session).
But more importantly - WHY do the numbers differ?
Google Analytics discounts Bounces (or Single Page View Visits as the new WAA standard definitions calls it) and IndexTools includes ALL visits into the calculation. So to be clear, if we have:
1000 visits
300 bounces
2300 minutes spent on site
Remember that page tagging solutions like IndexTools and Google Analytics do not count time spent on site for bounces (Single Page View Visits). Thus in the above example the 2300 minutes are spent on 1000-300 = 700 visits. Even though we know for a fact that those 300 bounces (Single Page View Visits) in fact did spend time on the site.
- IndexTools = 2300 minutes / 1000 visits = A Visit Duration Average on 2.30 minute. (or Avg. Time per Visit 2m 30s as we would write it)
- Google Analytics = 2300 minutes / 700 visits = A Visit Duration Average on 3.29 minute. (or 00:03:29 Avg. Time on Site as my friends at Google write it)
So NOW the questions goes, who is right and who is wrong? :-)
In point of fact, I think both calculations are justified and I see a reason to include both of them. It might be a bit clumsy to start pre-segmenting visits (visits with MORE than 1 page view) as our friends from Google Analytics have done (sorry Avinash and Brian). Looking at a calculation where we discount bounces, we are actually just saying that we want to look at the, to some extent, more engaged users (those who decided to dig further into the site). I therefore prefer the original calculation (as Google Analytics used to have it as well) where we look at ALL visits. Then one can always segment to separate more or less engaged users from the others, whether that is page views or not.
As an example. Let us look at May 2007 (I can of course recreate these results with on the fly segments):
1 page views or more segment = 04.11 (the complete data set)
2 page views or more segment = 10.54
3 page views or more segment = 17.38
4 page views or more segment = 22.30
As you can see the more page views people look at the more time they spend on the site. And all of a sudden starting on 2 page views or more seems awkward.
Conclusion (this is for you guys at the WAA):
If you add another definition to your standard set about visit duration, I suggest you call it: “Visit Duration Average” and the calculation is the total length of time spent in all visits (sessions) divided by the total number of visits (session). Hereafter one can segment on page views or more intelligent engagement metrics.
Further fine comments on the subject: