Web Analytics Tutorial

Appendix B – Technical Details of Metrics Accuracy

Visit Time Issues

In the introduction to this lesson, we discussed some of the problems with Visit Detection. Because HTTP requests are distinct, there is no absolute way of determining which request comprise a single visit. In addition, Proxies add another factor by obscuring the source of the requests and making it hard to distinguish different users who may be visiting at the same time.

View Time

Summary tracks the time a particular page was viewed and includes this column in some reports. The Average View Time and Total View Time reports are the primary resource for this. For web analytics to estimate how long a visitor was viewing a page, it generally looks at the time between the request to that page and the next one in a visit. However, the requests that make it to the server may not include all the pages viewed. Browser caches and web caches will deliver pages to users that never register on your site. So a user could browse three pages on your site, then return to a previously viewed page or two, then continue to a new one. Between the third and last, there is no request information on your server and the entire time between the requests is counted as view time for the third page. If a user has several browser windows open at once, he could be viewing another site during the time between requests to your site. If you have pages that automatically refresh themselves (because they host advertisements or contain continually updating data) then these will obviously have an average view time of about the refresh rate, which may not correlate to the time a visitor actually viewed the page.

Finally, the last page of a visit is where the user either clicks a link to a new site, types in a new URL or closes the browser. In most cases, you have no record of when she left the site. This means that Summary cannot determining how long the last page view was. In the case of pages that contain extended information that a user was searching for, this could be a significant amount of time that is never recorded.

Visit Duration

This last point becomes especially important when trying to calculate the length of the entire visit. Because there is no record of when the visitor left the site, visit duration, as summarized in the Duration of visit, does not include the time the visitor spent on the last page of the visit.

Fortunately, there is a partial solution to this. In Lesson 7 - Determining Visitor Behavior Patterns we talked about setting up a script or configuring your web server for link following. When you do this, each link that leaves your site goes through a redirector first. This means that you get one last request in your log files as the visitor is leaving. If you install this script then you can track the view time of exit pages and your visit duration will include the entire length of the visit for visitors following links to another site. Some visitors will close their browser or type in a new URL rather than following a link. The only way to track this is to install some JavaScript code on every page of your site that makes a final request to your site before the page changes.