Google has changed its secret search formula in the United States to be more discerning when it comes to which websites are worth recommending and which should sink in the rankings.
The move announced late Thursday was part of an ongoing duel between the search titan and low-quality websites that feature only content copied from elsewhere on the Internet or use techniques to trick their way high in results.
"Many of the changes we make are so subtle that very few people notice them," Google principal engineer Matt Cutts and Google fellow Amit Singhal said in a blog post.
"But, in the last day or so we launched a pretty big algorithmic improvement to our ranking."
They said the search formula change affects 11.8 percent of search queries, dropping low-quality websites in results while elevating high-quality websites with original content such as research, analysis, or in-depth reports.
"We do have a responsibility to encourage a healthy web ecosystem," Cutts and Singhal said.
"Therefore, it is important for high-quality sites to be rewarded, and that's exactly what this change does."
Google said that it has been the issue for more than a year and engineers spent several months working on the algorithm change, which was implemented in the United States and will be rolled out in other countries over time.