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The importance of a page is measured by the number and quality of links pointing at it. The degree to which Google trusts a site's inbound links also influences the importance of a page. If Google detects paid links, for example, it will devalue the links or nullify them completely so that PageRank will not pass to the target page.
A supplemental page will still rank in search results, but only if there are not enough pages in the main index that are returned within the search.
Google used to place a "Supplemental Result" label at the bottom of a search result to indicate that it is in the supplemental index; however in July 2007 they discontinued this practice and it is no longer possible to tell whether a result is in the supplemental index or the main one.  .
Causes of Supplemental Results
Some people believe that the supplemental index is Google’s way of filtering out duplicate content. TITLE elements, META descriptions tags, and navigational text that are similar or identical can lead to duplicate or near-duplicate content.
However, duplicate content is a side effect of supplemental results, not the cause. Instead, low PageRank is the primary cause of supplemental results. Links to multiple versions of the same page dilutes PageRank across multiple URLs, thus increasing their chance of not reaching a minimum PageRank threshold. If a page's PageRank is too low, Google will drop it from its main index. That page will appear in search results as a supplemental result.
Lack of Trust
Manipulative linking practices can also lower PageRank flow into a domain, thus creating more supplemental pages. Manipulative links include excessive reciprocal linking, link injections, and paid links.  Questionable outlinks can also lead to link devaluation. 
High Page Count
A large site with a high page count is also generally more vulnerable to the supplemental index than a small site because inbound PageRanks divided among several hundred thousand pages tend to be lower than that divided up among only a few dozen pages.
Page freshness is also a factor.
Recommended Solutions to Supplemental Results
The primary cure for supplemental results is more quality backlinks. Other solutions include internal links restructuring and trust management.
- ↑ Google Technology
- ↑ Search Engine Spam? Matt Cutts: those links ... have not been trusted in terms of linkage for months and months.
- ↑ How to Fix Supplemental due to Duplicate Content
- ↑ Deftly Dealing with Duplicate Content Adam Lasnik explains how to tackle duplicate content issues.
- ↑ Buffy in Duplicate Ex-Googler Vanessa Fox on why duplicate content is not the cause of supplemental results.
- ↑ Indexing Timeline Matt Cutts, a Googler, explains how manipulative links can create supplemental results.
- ↑ Condemned To Google Hell Forbes names a supplemental results victim.
- ↑ Being Condemned to Google Hell and Matt's Rebuttal Matt Cutts names the site owner as the culprit.
- ↑ Indexing Timeline Matt Cutts, a Googler, explains how a real estate site linking out to mortgages sites, credit card sites, and exercise equipment went from 10K pages indexed down to 80 after Big Daddy rollout.
- ↑ SMX Video - Matt Cutts Explains How to Get Out of Google's Supplemental Index Matt Cutts explains the relationship between PageRank, site size, and supplemental results during a session at Seattle SMX 2007.
- ↑ Getting into Google According to Jill Whalen, Dave Crow, Google's director of crawl systems, stated during SEMNE July 2007 that page freshness is a factor in whether or not a page is put into the supplemental index.
- ↑ Skinny on the Supplemental Index Adam Lasnik, a Googler, explains how to get out of the supplemental index.
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