Google “personalizes” as much as 20 per cent of your web searches, according to Mountain View software engineer Bryan Horling.

Speaking this afternoon at the search-obsessed SMX West conference in Santa Clara, Horling – a developer with Google’s personalized search team – said that up to one in five searches are tailored to the user’s particular location, web history, or online contacts. “As it stands today,” Horling explained, “between these three techniques, just about every user who’s engaging with Google search today is affected.”

Article from: http://www.theregister.co.uk/2010/03/03/google_personalized_search_explained/

This does not include search results that Google tailors to particular countries or continents. If you search for “taxes,” for instance, results in the US will differ widely from those in the UK. In the US, you’ll get info from the IRS. In the UK, you’ll get stuff from HM Revenue & Customs Page.

Google has tweaked results from country to country for years. Now, it’s tweaking results based on the individual user’s behavior. But Horling emphasized that many of these changes are rather subtle. “When these techniques fire, the changes tend to relatively minor,” he said. “We’re moving a few results. We might be moving a few down. We’re generally not changing the entire character of the page.”