Well, let’s talk about when you are logged in, because that’s a much easier case to talk about. If we know where you are located, whether you’re using a mobile phone, or whether you’re using a web browser, we can return you better results. If you type in pizza, or yoga instructor, or plumber, you’d much rather see the ones that are close by than something that’s located in San Francisco if you are in New York, or vice versa.
Even down to the city level, to try to say: ‘Oh, this person is in this city. Let’s try to show them relevant, useful, results from nearby. Previous searches, in the same way, we’ve been very clear; if you sign in, we try to personalize your search results. And that can be very helpful because if you know that ATM is a networking protocol in addition to, you know, a cash machine, maybe you are more interested in the Asynchronous Transmission Protocol sort of stuff.
Now, if you want to clear your cookies, you are more than welcome to. And in that case, you know, we don’t know anything about you. Maybe your IP address can tell us a little where we are guessing your country, and those sorts of things. But most people, the vast majority of people, seem to say and be OK with the idea that, OK, if I’m willing to tell Google a little bit of information, the quality of the search results can be better for me.
by Matt Cutts - Google's Head of Search Quality Team