Google said late Thursday that it had made a major change to its algorithm in an effort to improve the rankings of high-quality Web sites in its search results — and to reduce the visibility of low-quality sites. While the company did not say so explicitly, the change appears to be directed in part at so-called content farms like eHow and Answerbag, which generate articles based on popular search queries so they will rise to the top of the rankings and attract clicks.

Google has been facing criticism from some users for allowing articles that aren’t useful to appear prominently in search results. That has now changed, according to the company.

“This update is designed to reduce rankings for low-quality sites — sites which are low-value add for users, copy content from other Web sites or sites that are just not very useful,” Amit Singhal, a Google fellow, and Matt Cutts, who leads Google’s spam-fighting team, wrote in a company blog post. “At the same time, it will provide better rankings for high-quality sites — sites with original content and information such as research, in-depth reports, thoughtful analysis and so on.”

Google has said that it makes about 500 changes to its algorithm a year, but most are so small that the company doesn’t announce them. This one will affect 11.8 percent of search queries, Google said, so it is big enough to significantly change the results that people see.

Google’s announcement did not mention content farms. But Mr. Cutts has spoken in recent weeks about the problem and said Google was working on algorithm changes to fix it. “In general, there are some content farms that I think it would be fair to call spam, in the sense that the quality is so low-quality that people complain,” he said in a recent interview.

While the content on these sites can be useful, much of it is closer in value to the eHow article on making friends in college, which includes tips like “consider joining a sorority or fraternity” and “remember to have a good time, smile and laugh.” Many of the articles on these sites are phrased as how-tos, and even after Thursday’s change they still show up as top results in searches for how to do something — if someone phrases a search query that way, they might want to read such an article. In the blog post, Google said the updated algorithm rewards high-quality sites, so the effect will become clear over time.

Danny Sullivan, editor of Search Engine Land and an industry expert, said Google’s action “does seem to be both a P.R. move and a move to improve relevancy.”

Last week, Google introduced an extension to its Chrome browser that people can use to block certain sites from appearing in search results, and said it would study which sites people block to figure out which ones bother users. On Thursday, Google said that it did not use this data to change the algorithm, but that the new algorithm caught 84 percent of the most-blocked Web sites.