freejung - 8:07 pm on Jun 28, 2011 (gmt 0)
I just had a look at some of my competitors' Main Index Ratios, and they're all over the place.
It appears to me that it correlates more closely with site size than with ranking. My site is small (~2000 pages total indexed, ~1000 in main index). A competitor with 120,000 pages is at about 10%. I don't know whether he was pandalized, but his rankings for the keywords I track seem to have gone down somewhat. Others with only a few thousand pages are at or above 50%, whereas others with hundreds of thousands of pages are way down, usually below 10%.
I bet the closest correlation you would find with this metric would be homepage-PR/#-of-pages, which is quite consistent with the way indexing seemed to work pre-Panda.
No offense, but I think it's a red herring.