How does Google Analytics calculate $Index?

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Assume that a visitor to your website follows this path:

Page 1 >
Page 2 >
Page 3 >
Page 2 >
Page 4 >
Goal 1 Page >
Page 5 >
Transaction Page >
Exit

When Google Analytics examines the path to determine the $Index value for each page in the session it removes the duplicate hit to Page 2. So, our new session path will appear as:

Page 1 >
Page 2 >
Page 3 >
Page 4 >
Goal 1 Page >
Page 5 >
Transaction Page >
Exit

Goal 1 is defined as having a value of $10 and the Transaction has a value of $15.

In our example we can say that Page 1, Page 2, Page 3 and Page 4 all led to the user reaching Goal 1 and completing the transaction. Because the user hit each of the pages on their way to completing the goal and the transaction each page is assigned a $Index value of $25 ($10 for the goal + $15 for the transaction).

Page 5 is given a value of $15 because it only led to the completion of the transaction.

So, if our Google Analytics reports only contained data for the single session above then we would have the following $Index data:

Page Name $Index
Page 1 $25
Page 2 $25
Page 3 $25
Page 4 $25
Page 5 $15

Your site will have more than one session (hopefully!). To calculate $Index across all sessions Google Analytics cross examines all sessions, analyzing the click paths and goals/transactions completed for each session. It them calculates $Index based on the revenue that a page helps generates and the number of sessions that the page appears in. In formula form, $Index is calculated as follows:

total revenue page X helps generate / number of sessions that include page X = $Index

Let’s go back to our example. If there were nine more sessions that hit Page 3, and none of those sessions reached a goal or completed a transaction, then the value of $Index for Page 3 would be $2.50 ($25 revenue from Page 3 / 10 sessions including Page 3 = $2.50).

The power of $Index is that is tells you which pages lead to conversions on your website and which pages don’t. Now you can start to optimize the low value $Index pages and keep the high value $Index pages in your conversion paths.