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alan.blog-city.com: An evening with Googles Marissa Mayer

4. The infamous “I feel lucky” is nearly never used. However, in trials it was found that removing it would somehow reduce the Google experience. Users wanted it kept. It was a comfort button.
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8. They use the 20% / 5% rules. If at least 20% of people use a feature, then it will be included. At least 5% of people need to use a particular search preference before it will make it into the ‘Advanced Preferences’.