By Jonathan Fildes
Science and technology reporter, BBC News
People's movements were not as random as predicted
The whereabouts of more than 100,000 mobile phone users have been tracked in an attempt to build a comprehensive picture of human movements.
The study concludes that humans are creatures of habit, mostly visiting the same few spots time and time again.
Most people also move less than 10km on a regular basis, according to the study published in the journal Nature.
The results could be used to help prevent outbreaks of disease or forecast traffic, the scientists said.
"It would be wonderful if every [mobile] carrier could give universities access to their data because it's so rich," said Dr Marta Gonzalez of Northeastern University, Boston, US, and one of the authors of the paper.
Dr William Webb, head of research and development at the UK telecoms regulator, Ofcom, agreed that mobile phone data was still underexploited.
"This is just the tip of the iceberg," he told BBC News.
Researchers have previously attempted to map human activity using GPS or surveys, but it is expensive.
One innovative approach tracked the movement of dollar bills in an attempt to reconstruct human movements.
The study used data from the website wheresgeorge.com, which allows anyone to track a dollar bill as it circulates through the economy. The site has so far tracked nearly 130 million notes.
All of the mobile phone data was collected anonymously
Studies such as this suggested that humans wander in an apparently random fashion, similar to a so-called "Levy flight" pattern displayed by many foraging animals.
However, Dr Gonzalez and her team do not believe this approach gives a complete picture of people's movements.
"The bills pass from one person to another so they can't measure individual behaviour," she explained.
The new work tracked 100,000 individuals selected randomly from a sample of more than six million phone users in a European country.
Each time a participant made or received a call or text message, the location of the mobile base station relaying the data was recorded.
The researchers said they were "not at liberty" to disclose where the information had been collected and said steps had been taken to guarantee the participants' anonymity.
For example, individual phone numbers were disguised as 26 digit security codes.
"Furthermore, we only know the coordinates of the tower routing the communication, hence a user's location is not known within a tower's service area," they wrote.
Each tower serves an area of approximately 3 sq km.
Information was collected for six months. But, according to the researchers, a person's pattern of movement could be seen in just three.
"The vast majority of people move around over a very short distance - around five to 10km," explained Professor Albert-Laszlo Barabasi, another member of the team.
"Then there were a few that moved a couple of hundred kilometres on a regular basis."
The results showed that most people's movements follow a precise mathematical relationship - known as a power law.
Nokia believes phones could be fitted with sensors to collect data
"That was the first surprise," he told BBC News.
The second surprise, he said, was that the patterns of people's movements, over short and long distances, were very similar: people tend to return to the same few places over and over again.
"Why is this good news?" he asked. "If I were to build a model of how everyone moves in society and they were not similar then it would require six billion different models - each person would require a different description."
Now, modellers had a basic rule book to follow, he said.
"This intrinsic similarity between individuals is very exciting and it has practical applications," said Professor Barabasi.
For example, Professor John Cleland of the London School of Hygiene and Tropical Disease (LSHTM) said the study could be of use to people monitoring the spread of contagious diseases.
"Avian flu is the obvious one," he told BBC News. "When an outbreak of mammalian infectious airborne disease hits us, the movement of people is of critical concern."
Dr Gonzalez said that traffic planners had also expressed an interest in the study.
Although the scale of the latest study is unprecedented, it is not the first time that mobile phone technology has been used to track people's movements.
Scientists at MIT have used mobile phones to help construct a real-time model of traffic in Rome, whilst Microsoft researchers working on Project Lachesis are examining the possibility of mining mobile data to help commuters pick the optimum route to work, for example.
Location data is increasingly used by forensic scientists to identify the movements of criminal suspects.
For example, the technique was used by Italian police to capture Hussain Osman, one of four men jailed for the failed suicide bombings in London on 21 July.
Commercial products also exist, allowing parents to track children or for friends to receive alerts when they are in a similar location.
These types of services and projects will continue to grow, Dr Webb believes, as researchers and businesses find new ways to use the mobile phone networks.
"There are so many sensors that you could conceivably attach to a phone that you could do all kinds of monitoring activities with," he said.
For example, Nokia have put forward an idea to attach sensors to phones that could report back on air quality. The project would allow a large location-specific database to be built very quickly.
Ofcom is also planning to use mobiles to collect data about the quality of wi-fi connections around the UK.
"I am sure there will be tens if not hundreds of these ideas emerging over the next few years," said Dr Webb.