Agricultural economists sometimes have a choice of using either unit values or community prices when analysing food price policy. Unit values (ratios of household expenditure on a food to the quantity purchased), and community prices, which are enumerated from vendors in local markets, are both proxies for market prices. While it is believed that biases may result from the use of unit values, due especially to measurement error and quality effects, evidence on this issue is lacking. Even less is known about community prices. This paper provides empirical evidence that suggests that economists should exercise caution when using unit values as proxies for market prices. Community prices in our two case studies have a number of properties that make them more reliable in our two case study countries: Vietnam and Papua New Guinea. Price elasticities calculated from unit values provide poor approximations to those calculated with community prices. If the unit value-based (or community price-based) elasticities are biased, they may distort food policy analysis. In our study, the use of unit values (if wrong) could lead policy makers to decide to liberalize rice exports which could generate unexpected adverse nutritional consequences because price elasticities were understated.