5:55 p.m. | Updated with additional details.

SAN FRANCISCO — Google, acknowledging that some searches were giving people stale results, revised its methods on Thursday to make the answers timelier. It’s one of the biggest tweaks ever to Google’s search algorithm, affecting about 35 percent of all searches made.

The new algorithm is a recognition that Google, whose dominance depends on providing the most useful results, is being increasingly challenged by sites like Twitter and Facebook, which have trained people to expect to be constantly updated with seconds-old news. It is also a reflection of how people are using the Web as a real-time news feed: if, for example, you search for a baseball score, you probably want to find out the score of a game being played right now, not last week, which is what Google often gave you.

“This is the result of them saying, ‘We need to find a way to more effectively get fresh content up,’” said Danny Sullivan, editor of Search Engine Land and an industry expert. “It does help with the issue of people thinking, ‘Wow, if I need to find out about something breaking I’ll go to Facebook or Twitter for that.’”

Google tried before to create real-time search, in 2009, when it introduced google.com/realtime, a service that incorporated Twitter posts that Google paid Twitter to use. But that contract expired in July and the two companies could not agree on terms to renew it, so Google disabled the site.

For Internet users who want the latest chatter about events happening now, Google competes with Facebook, Twitter and Bing, Microsoft’s search engine, which unlike Google, includes Twitter and Facebook posts in search results.

“The biggest source of the very freshest information is Twitter, and Google doesn’t have anywhere near the access to that kind of data as it had before,” Mr. Sullivan said. “But when people do those kinds of searches, they’re looking for a lot of reactions, looking for Twitter itself. So even with these changes, this doesn’t really solve that problem.”

Google became dominant by surfacing archived Web sites. But today people sometimes expect years-old links, like the best banana bread recipe; week-old links, like the last episode of Gossip Girl; or seconds-old links, like this morning’s presidential election news.

Google makes more than 500 changes to its algorithm a year, but most affect only a small percentage of results. With its new formula, which Google calls a freshness algorithm, Google tried to teach itself the difference between those types of requests, Amit Singhal, a Google fellow who works on search, wrote in a blog post announcing the changes.

“Depending on the search terms, the algorithm needs to be able to figure out if a result from a week ago about a TV show is recent, or if a result from a week ago about breaking news is too old,” he wrote.

“This algorithmic improvement is designed to better understand how to differentiate between these kinds of searches and the level of freshness you need.”

Google last announced a significant change to its search algorithm in February, when it said it would raise the rankings of high-quality sites in an effort to fight low-quality ones, often described as content farms, that were flooding the search engine with mindless articles about popular search queries. Though Americans still turn to Google for two-thirds of their Web searches, it faces competition from Bing, and from nonsearch sites like Facebook that cater to people who want immediate answers from people they know.

The new formula, which affects search results globally but will not change ads that appear on Google, will bring up minutes-old results for recent events, like an unfolding news story, and for recurring events like the Oscars or the presidential election. It will also show fresher results for topics that are often updated, like reviews of a new iPhone, but will understand that unlike breaking news, reviews from a few weeks ago are also useful, the company said.

These are “queries we don’t think we’re doing perfectly well on,” said Rajan Patel, a Google software engineer who worked on the new algorithm. “We just realized that people expect Google to return the most up-to-date results for all kinds of queries, from hot topics to more general queries like a T.V. show.”

For evergreen results, like recipes or how to change a tire, Google said the algorithm would know to show the best results no matter when they were posted.

The algorithm uses technology that Google built last year, in response to the speed at which people were publishing real-time updates online. It is a Web indexing system it calls Caffeine, which crawls the Web more quickly, updating Google’s index of Web sites continuously instead of every couple of weeks. Thursday’s revision changes how Google ranks those links now that it has them in its index, Mr. Patel said.

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