Do you know what supplemental results means, how it works, and why it could be very bad news for you? If not, you need to ensure that your rankings are secure.
You see, every time Google indexes a page, it checks to see if the page is already in its database. Duplicate content or pages which have not been accessed for a long time can then be labelled as a as a supplemental result. As you can imagine, many webmasters hate being classed this way, since it means they lose obvious rankings for that site.
A website being relegated to the status of supplemental result almost always has to do with Google perceiving duplicate content or design elements.
Duplication of content used to mean single websites producing identical pages by the dozens and posting them on their site to try to trick search engines into thinking that had X number of keyword optimized pages. This temporarily improved the keyword density of some websites, driving up its chances of coming up multiple times for the same keyword, but Google soon caught on and plugged the leak.
Now, as soon as Google detects identical content, it automatically classes all the similar content as a supplemental result – not only the duplicate pages, but the even original page or website can suddenly become a supplemental result!
Another method that can lead to a designation of supplemental content is giving all the pages the same heading or assigning similar attributes. This can (often incorrectly) cause Google to detect that the same page name has been used with different content (which is another old SEO technique that no longer works.) This shows that the term duplication applies not just to content, but also similar page or design structure!
Blog sites and forums are not reported as supplemental results – apparently Google can tell which fall under this classification and which are websites simply abusing the power of content. One more reason that Google might mark sites or pages as supplemental results is when the site receives very low traffic. Google might assume that if a page that has not been visited in months, it’s probably not useful. This leads to the label as a supplemental result, but these types of supplemental results are rare and easy to correct simply by increasing traffic and incoming links.
If your site has been classified as a supplemental result, remove any duplicate content or duplicate web structure. You can also redirect pages, no-index pages, and increase traffic to quickly get your ranking back.