How three small credit card transactions could reveal your identity

A new study suggests anonymous data sets aren't very anonymous after all

Just three small clues - receipts for a pizza, a coffee and a pair of jeans - are enough information to identify a person's credit card transactions from among those of a million people, according to a new study.

The findings, published in the journal Science, add to other research showing that seemingly anonymous data sets may not protect people's privacy under rigorous analysis.

"The fact that a few data points are enough to uniquely identify an individual was true in credit card metadata," said Yves-Alexandre de Montjoye, an MIT graduate student and one of the study's authors.

Montjoye and his colleagues analyzed credit card transactions provided by an unnamed major bank from 1.1 million people over a three-month period in some 10,000 stores.

They were trying to see how much data they needed to identify a person's transactions from a larger set of transaction records. Absent from the data were names, addresses, email addresses and other personal information.

Ninety percent of the time, the researchers could pick out an individual using just four pieces of data, such the locations where four purchases were made. Adding price information -- for example, purchase receipts -- allowed them to identify a person with just three transactions.

They could also identify individuals from "one receipt, one Instagram photo of you having coffee with friends, and one tweet about the phone you just bought," they said.

"The fundamental scientific question is one of our human behaviour," de Montjoye said. "It's really how our behavior compares with that of others and eventually makes us unique and identifiable."

The researchers didn't try to actually identify particular individuals, but instead to figure out on average how much data would be needed to narrow transactions down to a person.

"We did not try to find a specific person on purpose," he said.

The latest research adds to a 2013 study de Montjoye co-authored that showed that four data points, such as place and time, were enough to identify a person from a mass of mobile phone records 95 percent of the time.

The research highlights the regulatory and policy challenges around anonymity, de Montjoye said. Legally, society relies on a definition of anonymity -- such as removing names and email addresses from records -- that is widely believed to provide protection.

"What our study shows is that this is not enough to prevent identification," he said.

The other way to define anonymity, endorsed by the European Union, is that data must be "provably" anonymous, and make it impossible to identify an individual under any circumstances.

Verifying that condition is difficult, de Montjoye said. In addition, scrambling the data too much may prevent novel and legitimate uses, such as studying consumption patterns or inflation. But people should be aware of the potential risks of identification.

"I don't think it's ever going to be 100 percent safe, but there are steps that can be taken," he said.

Send news tips and comments to Follow me on Twitter: @jeremy_kirk

Join the PC World newsletter!

Error: Please check your email address.

Tags securityMassachusetts Institute of Technologyprivacy

Our Back to Business guide highlights the best products for you to boost your productivity at home, on the road, at the office, or in the classroom.

Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.

Jeremy Kirk

IDG News Service
Show Comments

Most Popular Reviews

Latest News Articles


GGG Evaluation Team

Kathy Cassidy


First impression on unpacking the Q702 test unit was the solid feel and clean, minimalist styling.

Anthony Grifoni


For work use, Microsoft Word and Excel programs pre-installed on the device are adequate for preparing short documents.

Steph Mundell


The Fujitsu LifeBook UH574 allowed for great mobility without being obnoxiously heavy or clunky. Its twelve hours of battery life did not disappoint.

Andrew Mitsi


The screen was particularly good. It is bright and visible from most angles, however heat is an issue, particularly around the Windows button on the front, and on the back where the battery housing is located.

Simon Harriott


My first impression after unboxing the Q702 is that it is a nice looking unit. Styling is somewhat minimalist but very effective. The tablet part, once detached, has a nice weight, and no buttons or switches are located in awkward or intrusive positions.

Featured Content

Latest Jobs

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?