Advance Funnel design considerations
The key point of the Funnel report is to show the tendency of specific page groups to lead to progress to 'deeper' stages in the funnel. Each page group has a percentage assigned to it and a matching color. This percentage represents the number of sessions where the request for a page group was followed by a request for a page group in a deeper stage divided by the number of visitor sessions in which that page group was requested.
So if a page group is requested in 100 visitor sessions, and in 75 of those sessions a visitor subsequently visited a deeper stage, that page group would have a progression rate of 75%. So far so simple. But now let's look at some of the subtleties involved.
Let's say that you've designed a funnel with three stages (A, B, and C), and two page groups in each of the first two stages. So you have page groups A1, A2, B1, B2, and C (the goal page). The visitor follows this path:
A1 > B1 > B2 > C > A2 > exit
How would this one page factor in to the statistics about progressions? Clearly A1, B1, and B2, would get credited with a progression. What about A2? The page view was not followed by any deeper page views, so you really can't count it as a progression. On the other hand, the visitor did see the deeper page groups within that session, which indicates that they were interested in both pages. It doesn't seem right to call this a failed page group.
In cases like this, we split the difference by not counting this session at all in terms of that particular page group (A2 in this case). So if prior to this session the page group had 10 visits of which 5 had progressions, and we counted it against the page group, it would be 5 of 11, thus dropping its ratio. If we counted a progression, it would be 6 of 11, which would raise the ratio. By not counting the session (again, just for this page group), we leave the ratio as-is at 5 of 10.