In personal digital gadgets, for example, 23 percent of respondents said they had bought such goods for the first time.

This data set, created by shoppers themselves using in-store scanners, records the type of goods bought by various income groups between 1994 and 2005, as well as the prices paid for them.

Big retailers, in turn, complain that suppliers impose geographical barriers: identical goods produced in the same factory must often be bought at different prices to be sold in different countries.