Google says it has tweaked the formulas steering its Internet search engine to take the rubbish out of its results.

The overhaul is designed to lower the rankings of what Google deems "low-quality" sites. That could be a veiled reference to so-called online "content farms" such as Demand Media's eHow.com.

Sites that produce original content or information that Google considers valuable are supposed to rank higher under the new system.

The change announced late Thursday affects about 12 percent, or nearly one in every eight, search requests in the U.S.

Google Inc. says the new ranking rules eventually will be introduced in other parts of the world, too.

News of Google's move caused Demand Media Inc.'s shares to fall nearly 5 percent in afternoon trading Friday.