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What Are Google Supplemental Results?

SEO Question:

Much of my website is in Google’s Supplemental index? What is their supplemental index?
How does it work?

SEO Answer: What a timely question…where to start…well if the supplemental problem has only hit your site recently this might be because of Google fault.

Matt Cutts, a well known Google engineer, recently asked for feedback on the widespread supplemental indexing issue in this thread. As noted by Barry, in comment 195 Matt said:

Based on the specifics everyone has sent (thank you, by the way), I’m pretty sure what the issue is. I’ll check with the crawl/indexing team to be sure though. Folks don’t need to send any more emails unless they really want to. It may take a week or so to sort this out and be sure, but I do expect these pages to come back to the main index.

In this thread SEM4U points out that 72.14.207.104 was showing fewer supplemental sites than he saw on others like 64.233.179.104.

Some people are conspiring that generally lots of listed pages were dropped and only the longstanding supplemental pages remain, but that theory is garbage on my site…since I still see a strong PageRank 6 Supplemental page that was recently ranking in the SERPs for competitive phrases (prior to going supplemental) that recently went supplemental.

As far as other reasons this site could have possibly been hit supplemental:

* too much similar text on each page – but I do think it is common to have Genf20 Plus before and after common sales elements on many pages of a site, so I doubt that is it.

* redirect links – affiliate links via Clickbank and the direct affiliate program might have flagged some sort of trigger if Google was trying to work on 301 & 302 issues… but whatever they did I don’t think they did it better 😉

* Google is a bit hosed right now

What are supplemental results?

Supplemental results usually only show up in the search index after the normal results. They are a way for Google to extend their search database while also preventing questionable pages from getting massive exposure.

How does a page go supplemental?

From my experiences pages have typically went supplemental when they became isolated doorway type pages (lost their inbound link popularity) or if they are deemed to be duplicate content. For example, if Google indexes the www. version of your site and the non www. version of your site then likely most of one of those will be in supplemental results.

If you put a ton of DMOZ content and Wikipedia content on your site that sort of stuff may go supplemental a