STARTING in February, Jeffrey G. Katz grew increasingly anxious as he watched the steady decline of online traffic to his company's comparison-shopping Web site, Nextag, from Google's search engine.

In a geeky fire drill, engineers and outside consultants at Nextag scrambled to see if the problem was its own fault. Maybe some inadvertent change had prompted Google's algorithm to demote Nextag when a person typed in shopping-related search terms like "kitchen table" or "lawn mower."

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