A day before celebrating its 15th birthday, Google Inc. announced it had overhauled the search engine algorithm that performs millions of searches daily. The latest change in the way the ubiquitous Web company serves up results to users seeking information online could have a major impact on traffic to everyone else’s websites, according to some experts.

The redesign, which represents the most dramatic alteration to the search engine in three years, will affect the analysis of about 90 percent of the search requests that Google gets, according to Amit Singhal, a senior vice president for the Menlo Park, Calif.-based company.

Google disclosed the existence of the new search formula Thursday at an event held in the garage where CEO Larry Page and fellow co-founder Sergey Brin started the company 15 years ago.

Any reshuffling of Google’s search rankings can have sweeping ramifications because the search engine steers so much of the Internet’s traffic, fielding about 2 out of every 3 search requests in the U.S.

The changes could also drive up the price of Google ads tied to search requests if websites whose rankings are “demoted” under the new system feel they have to buy the marketing messages to attract traffic.

“Anytime there’s an update to Google, people panic,” said Andrew Garberson, search engine optimization lead of South Side-based digital marketing firm LunaMetrics.

A company will typically hire a digital marketing firm to help boost the visibility of its website in a search engine’s results. Search engine optimization, or SEO, efforts can improve a website’s organic, or nonpaid, ranking among those results; the higher the rank, the more people likely to see the website.

So if a customer searches for “fix broken sink Pittsburgh,” a plumbing company would want to make sure those keywords were in the copy on its website to enable the search engine, and the customer, to find it. Digital marketers advise companies on the most effective ways to attract search engine traffic.

Of particular concern for some marketers are apparent changes to Google’s analytics. Information on which keywords, or search terms, bring a visitor to a website is no longer available. So it will be harder for a company to determine what a customer searched for via Google that brought that customer to the company’s website.

“Before, you could have that short cut, that insight of someone’s search intent,” Mr. Garberson explained.

The latest changes to keyword information may force companies to return to more traditional — and costlier — forms of market research.

Search ads and other commercial pitches related to Web content account for most of Google’s revenue, which is expected to approach $60 billion this year.

The change is primarily aimed at giving Google’s search engine a better grasp at understanding concepts instead of mere words, Mr. Singhal said.

“It’s focusing less on keywords and more on the content,” said Paul Magnani, director of interactive at South Side marketing agency Gatesman + Dave. He said from an agency perspective, the changes put SEO in the hands of content writers more than it has been in the past.

“The better the quality of the content, the more originality in the content, the better off you are,” Mr. Magnani said. “But you have to balance that with the fact that the Web audience has very little patience: They don’t typically read, they scan.”

While time will tell how successful the changes are in improving search results, Mr. Magnani is doubtful the changes will come back to haunt the massive search engine operator.

“We live in Google’s world,” he said. “This is going to mean we have to understand our audience to the point where we understand the questions in their minds, not just the topics they’re interested in.”

Mr. Garberson said the changes to the search algorithm speak to Google’s core mission of improving user experience. “The idea is that cream rises to the top,” he said.

A search that better anticipates what a customer is trying to find, and that delivers better quality content, is a positive change, he said. “I don’t think the keyword is dead. But in a system that rewards value, having more components in a search helps.”

Kim Lyons: [email protected] or 412-263-1241. Twitter: @SocialKimLy. The Associated Press contributed.