What is a visit?Omniture says "a visit is a term that refers to a visitor’s access to a website. The visit begins when a person first views a page on your company's website. It will continue until that person stops all activity on the site for 30 minutes." You might want to view my previous post on the atoms of web analytics.
What is an instance?Again, Omniture says "an instance as relates to the number of times that a unique event occurs in SiteCatalyst". First, I wouldn't say in "SiteCatalyst"... the event occurs on my site, but that's a detail. An instance = count of event.
The counts doesn't match up!In the screen snapshot above, the number of visits is 62,013 while the number of instances for 87,444... How can SiteCatalyst register 41% more instances than visits? But wait! We're not measuring apples with apples. Instances and visits are two different measurement units.
Let's do an exampleNothing better than an example to demonstrate the difference.
|8:00pm||A visitor goes on Google and do a search for "web analytics".|
|8:01pm||That visitor click on a link to "immeria.net"||+1||+1 from Google|
|8:05pm||The visitor browse a few pages and leave my site.|
|8:06pm||The visitor goes back to Google and do a search for "Omniture 3rd party cookie" and find the second entry to be an article from my site||+0||+1 from Google|
|8:15pm||The visitor does a "back" and look for other results from Google, than find the fine piece about "Web Analytics technical implementation best practices".|
|8:30pm||After reading the article, the visitor look at the comments. One of them links to my site||+0||+1 from kaushik.net|
|9:30pm||The visitor leave for more than 30 minutes since the last hit to my site, then come back and click on my home page link.||+1|
Hope that helps!