Amazon, Yelp and similar Web sites rely on customer reviews to help users with their purchases. A nagging concern of shoppers, however, is how reliable these critiques are.

Bundle, a New York City-based start-up, has turned to a source it deems more objective: credit-card data. Bundle receives data on credit-card transactions from Citi, one of its investors. The data are stripped of personal details, but every cardholder is tagged with a unique identifier so spending can be tracked. The data also retain demographic information such as salary, marital status and household size. Bundle then compares each transaction against a commercially available list of 15 million merchants that accept credit cards, which includes the merchants’ locations. The idea is "to see people putting money where their mouth is," says Bundle CEO Jaidev Shergill. The company tracks such information as how many repeat customers a business has, the amounts customers usually spend, what types of people go there, and what other places customers of an establishment frequent.

The site went national with its data and its iPhone app in January, allowing users to find places they might like to check out, either in their hometown or someplace they are visiting, or to see what spending levels are like in cities to which they are thinking of moving. The site hasn't completely done away with the Yelp model of subjective feedback, however. It partners with companies that provide traditional qualitative reviews for a human touch, as well as to fill in gaps, such as businesses that only accept cash or American Express, which Citi does not track.

This article was published in print as "It Watches Your Wallet."