Google has given a glimpse on just how much information on its users it has collected – and who thinks they are.

However, it seems the famed Google algorithms are far from infallible.

People taking advantage of the facility that allows the public to view what kind of consumer Google thinks they are have been amused to find themselves listed with the wrong age and even sex.

Nevertheless, the knowledge that Google works so hard to profile its 350 million account holders is bound to intensify the debate about privacy which flared up again this week with the announcement that the company was going to start tracking users across all of its sites, including YouTube.

The detailed personal “profile” sums up many of a user’s interests, along with age and gender.

Google builds a detailed profile by harvesting the history of its account holders’ visits to sites in its advertising network.

User’s age and gender are decided by those of other Google users who have visited the sites you visit, leading to the mistakes.

One blogger from tech site Mashable found this week that Google’s Ad Preferences page assume that she was middle-aged – and a man, simply because her interests included technology and computing.

The profile page, called Ad Preferences, is hidden away inside a settings menu in Google Accounts, but can be accessed directly.

This sort of in-depth profiling raises alarm bells with privacy activists.

“Consumers have increasingly digital lives and they are developing an unfathomably large data trail every day,” says Rainey Reitman, activism director for privacy group Electronic Frontier Foundation.

“There has never been another time in history where privacy was under the kind of assault it is today.”

Uuser can opt out of the tracking, or manually edit his details. Google also does not store information on controversial subjects such as pornography.

The Ad preferences page came to public attention following a sweeping change to “privacy policy” which comes into effect on March 1, although the preferences page was launched some time ago.

YouTube data, Gmail information and search data will all be used to build up ever more accurate advertising profiles and also the company claims it will make searches more personalized.

In most cases, though, the data is eerily accurate, bringing up a breakdown of interests, age and sex.

The Advertising Preferences information that Google gathers is sent out as a “cookie” – a packet of information sent out by your browser – whenever you visit other Google partners, who then serve up “relevant” adverts when you visit their sites.

Users who are fearful of the amount of information Google holds can block the profiling by disabling “cookies” in their internet browser settings.

Google says: “We associate interests with your ads preferences based on the types of websites that you visit within the Google Display Network.”

“For example, when you browse many gardening-related websites in the Google Display Network, Google may associate a gardening preference with your cookie.”

“If the sites that you visit have a majority of female visitors, we may associate your cookie with the female demographic category.”

Some users reported that Google had identified bizarre interests such as “Sweets and Candy”, and was duly serving them adverts appropriate to that “interest”.

User can manually change his “interests” from your Ad Preferences page – although you cannot, of course, stop Google from sending you adverts.