When Advanced Segments in Google Analytics were introduced just over a year ago I was delighted. Finally there was an easy way to slice our website data in more than one dimension at a time. By setting up an advanced segment for just “Organic Visitors from the United States” for instance I could really drill down into what a specific set of users were doing on the site and see how their behaviour differed from the norm.

Every now and again though I used to see some strange results come back for reports I was doing. At the start I put this down to the feature being in Beta, but the anomalies didn’t go away. Having come back to use advanced segments in recent days to dig into some very specific user behaviour I noticed the problem was still there so I decided to find out what was causing it.

As an example, when I did a search (filter) in my top content report for all the URLs containing “/dentists/” I got a figure of 313,000 page views for the period of time I was looking at. However, if I swapped out the filter and used an advanced segment I had set up to do the same thing (i.e. only include data for pages that contained “/dentists/” in the URL) I got a figure of 434,000 page views, and I could see pages that clearly didn’t match the segment I thought I had setup.

At this stage the difference was far too big to ignore, and I assumed that lots of other users would be aware of it too. I was right. The Google Analytics help forum was full of questions about why data sliced with filters wasn’t matching data sliced by advanced segments.

The answer came in one line from a guy called MikeOstrowskiASU. He said simply that “Advanced segments are based on visitor sessions.”

Suddenly it was all clear. The advanced segments when used on the top content report were bringing back all the pages visited by anyone who had visited a page with “/dentists/” in the URL during their session rather than just the pages with “/dentists/” in the URL.

Just to confuse matters, advanced segments on visitor reports do match up with filtering because they are based on user sessions. For instance, using an advanced segment for “Visitors from the United Kingdom” on the visitors report will match up with going into the Map Overlay and clicking on the UK.

So, the moral of the story is be careful how you use advanced segments and what you infer from the data you get back. They might not be doing what you think they are.

Have you had a similar problem with Google Analytics, and have you found any workaround for it?