MICHELE NORRIS, Host:

So, what kind of information do search engines keep about all your searches. To find out, we turn to Danny Sullivan. He's the editor of SearchEngineWatch.com and Danny, before we begin, could you tell us exactly what Search Engine Watch does?

DANNY SULLIVAN: We track the search engine industry. We write about how search engines operate to educate both those who want to use them to locate information as well marketers who are trying to be found on them.

NORRIS: And so you'll be able to answer that question for us then. What kind of information do search engines keep about the people who use them?

SULLIVAN: To some degree, it depends on how you interact with them. If you simply go to Google, for example, each day and never signed up for any of their services, they will know that you came from a particular computer, a particular IP address, sort of your internet telephone number, and they may be able to, over time, know that you made certain kinds of searches. But they don't know who you are personally or anything like that.

If you sign up for one of their services, for example, they have a personalized search service, they'll start keeping a record of what you've searched for and that would be associated with your Google account which would be associated with your name and if you actually decided that you wanted to buy things like video from them as they sell, then that would be potentially associated with your credit card number and your actual address and they have a better idea of who you are in that way.

NORRIS: So are there differences between different search engines? Between Google and Yahoo, or Alta Vista, or any of, you know, there's so many search engines now, do they all keep the same kind of data?

SULLIVAN: They all seem to. I've not seen a breakdown in terms of say how long that they keep the data. So it may be that some of them are destroying some of the data after a certain period of time and other ones may be keeping it longer. Google's had the most attention in this area and they simply have seemed to have said we should go destroy the data and that may change.

It may be the people are going to start demanding that they do that, it may be that we'll have laws that will come in. But you do get some complications as well. The Google personalized search actually is a very nice service. It can learn the things that you've been to over time and it starts changing the kinds of search results you see, and I found that it actually can make your search even better. So, you as the searcher may want them to retain that data.

NORRIS: So you say they use this to perhaps tailor your searches. How else might they use that data?

SULLIVAN: The other way would be to deliver ads to you. If you take Yahoo! for example, if you've done a search on Yahoo, and then you go off and do other things, say you're over at Yahoo Movies and reading about some movie that's playing, but you had searched a day before for a new car. They might start showing you car ads because they know you are interested in cars based on your searching habits. So that's an example where they're targeting your search behavior maybe well after the fact of when you actually did that original search.

NORRIS: Well, before people learned about this Justice Department subpoena, do you think most people know that the search engines are compiling this kind of data.

SULLIVAN: No, I think it's still largely not aware among people and I don't know how much it will be a concern for some of the people as well. This type of thing exactly raises that kind of awareness because you don't really get a visibility of the search behavior that you're doing and that behavior needs to raise.

I mean people do need to be more aware of it because they have these conversations with search engines, very private conversations about things that they wouldn't tell a doctor, they might not tell a friend, they might not tell their spouses, but they are almost confessing to the search engine.

So you want to be aware of it and you want to be aware of it not just with what they are doing on the search engine, but even to the point that there's a record of what you're searching for on your own computer or even within your own browser. So, there are reasons why you may want to be protecting your privacy that way as well.

NORRIS: Danny Sullivan, thanks so much.

SULLIVAN: You're welcome.

BLOCK: Danny Sullivan is the editor of SearchEngineWatch.com. Transcript provided by NPR, Copyright NPR.