Google has made significant changes to its search algorithm in a bid to cut down on the amount of low-quality sites and spam returned in its search results, the company said on Thursday.
It says that the change will lead to different search results being returned in nearly 12 percent of all searches carried out.
"Many of the changes we make are so subtle that very few people notice them. But in the last day or so we launched a pretty big algorithmic improvement to our ranking," Amit Singhal and Matt Cutts said on the official Google Blog. "This update is designed to reduce rankings for sites which are low-value, copy content from other websites or sites that are just not very useful."
Conversely, Google said that it will result in better rankings for higher quality sites whose content includes research, analysis and other original content.
Google said that the refined algorithm doesn't use data collected by the Personal Blocklist Chrome extension launched on 14 February.
However, the change means that a lot of existing websites could find their Google ranking and placement for certain search terms shift overnight, Google said.
"We can't make a major improvement without affecting rankings for many sites. It has to be that some sites will go up and some will go down... It is important for high-quality sites to be rewarded," the company said.
Initially the rollout will be US-only but the company plans to extend it to other regions over time.
"We've been tackling these issues for more than a year, and working on this specific change for the past few months. And we're working on many more updates that we believe will substantially improve the quality of the pages in our results," the company said.