EVERY DAY HUNDREDS of Americans take to the streets to collect prices for the Bureau of Labour Statistics (BLS), the government’s economic-data agency. Enumerators in 75 urban areas track fluctuations in the costs of a “market basket” of goods and services, from blood tests at doctors’ offices to rental fees for two-bedroom apartments and cinema tickets. The numbers they gather feed into the Consumer Price Index (CPI), a statistical tool used to calculate inflation.