Google is changing the secret formula it uses to rank Web pages as it struggles to combat websites that have been able to trick its system.

Google handles nearly two-thirds of the world's Web searches, has been critisised recently over the quality of its results. Google said it has changed its mathematical formula late on Thursday in order to weed out "low-quality" sites that offer users little value. Some such sites offer just enough content to appear in search results and lure users to pages loaded with advertisements. Google has changed its search algorithm in an effort to filter out data from "content farms" in search results.

Google generates billions of dollars from advertising linked to its search engine, whose influence as a front door to the world's online content and commerce continues to grow by the year.

The Silicon Valley company built its business on the strength of algorithms that yield speedy results. The company constantly refines those formulas, and sometimes takes manual action to penalize companies that it believes use tricks to artificially rise in search rankings.

The problem is that content on the internet is growing exponentially and the vast majority of this content is spam," or of little or no use.

Google search engineer Amit Singhal said in an interview that the company added numerous "signals," or factors it would incorporate into its algorithm. Among those signals are "how users interact with" a site. It also used feedback from hundreds of people it regularly hires to evaluate changes. These "human raters" were asked to look at search results and decide whether they would give their credit card number to a site or follow its medical advice, On Thursday night, Mr. Singhal and a colleague wrote in a blog post that most of the changes would be "so subtle that very few people notice them" but "it's a big step in the right direction of helping people find ever higher quality in our results."

About 12% of U.S.-based queries would be affected by the change, Google said, and the changes would expand to non-U.S. users in the near future.

Overall Google has done a great job and there are very few cracks in the system, but spammers are getting smarter and Google needs to keep up with them.

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