Google Feb. 24 said it had flipped the switch on an algorithm change that pushes down low-quality Websites in its search engine, the latest in a series of moves to combat the rise of content farms and other Websites that infest the Web.

The ranking change, targeted at Websites that copy content from other Websites and those that provide little value for searchers, will impact 12 percent of the company's search results, said Google Fellow Amit Singhal and his lieutenant, Google principal engineer Matt Cutts.

Google didn't mention content farms by name, but Search Engine Land's Danny Sullivan posited the search engine could well be targeting sites such as Demand Media's eHow, which produces both solid content and low-quality content.

Demand Media responded to Google's change rather diplomatically in a blog post, noting that it hadn't seen any material net impact to its content business.

While the weak Websites will see their rankings drop, Singhal and Cutts said high-quality sites, or those with original content and information such as research, in-depth reports and analysis, should see better rankings.

The algorithm change currently impacts Google results shown only in the United States, though the company will eventually push it out in Google search in other countries.

The change comes 10 days after Google launched its Personal Blocklist Chrome extension, which lets people block Websites from their Web search results on Google.com. It's a sort of crowd-sourced approach to boosting search quality.

While the extension feeds Google information about blocked Websites Google may use as ranking signal, Singhal and Cutts said that the algorithm change did not leverage feedback from this tool.

But Google did compare the Blocklist data with the sites identified by its algorithm, and found that the preferences users "expressed by using the extension are well represented."

"If you take the top several dozen or so most-blocked domains from the Chrome extension, then this algorithmic change addresses 84 percent of them, which is strong independent confirmation of the user benefits," Singhal and Cutts said.

To wit, Google believes the algorithm change represents big search quality improvement, which couldn't come at a better time given the flak Cutts and his search quality colleagues have faced in recent months.

Cutts Feb. 1defended Google's search quality during a Microsoft Bing-sponsored search event, where he countered concerns about Google.com's search engine by accusing Bing of copying Google search results.

Cutts then revealed Feb. 12 that Google had to crack down on J.C. Penny after its SEO firm gamed Google to get top billing in dozens of popular product searches. Google executed a similar move against Overstock.com earlier this week.