A PDF they include (labeled Google Confidential and Proprietary in the footer) talks about a Google-initiated market study by Eye Square and TNS Infratest analyzing the efficiency of sponsored links on result pages. One slide includes an eye tracking study, showing where users focus on in results during the first 5 seconds; one major focus is the top yellow box showing the AdWords. Another slide tells advertisers how easily those AdWords are confused with organic search results. The following from an interview conducted with test user Erik is quoted by Google (my translation):

INT -interviewer-: Why do the results on top have a yellow background, did you notice?

TP -tester-: I didnt notice this.

INT: What does it mean?

TP: It definitely means theyre the most relevant.

3 years ago, Google co-founder Sergey Brin told Playboy in an interview: One thing thats important to us is the distinction between advertising and pure search results. We make it clear when something is paid for. Our advertising is off to the side and in a couple of slots across the top. Ads are clearly marked. Theres a clear, large wall between the objective search results and the ads, which have commercial influence. Other search engines dont necessarily distinguish.

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