Countries must therefore establish their own definition depending on their analytical needs, the objectives of the survey and the national context. Some countries have developed a classification of localities based not only on population size but also on the socioeconomic structure of the population in the localities and contiguous areas. Locality may coincide with a cluster in cluster sampling designs. Owing to the great variation among countries, questions aimed at measuring ethnicity, religion and language must be developed at the country level. The most recent census of the population may be useful as a starting point for developing these questions.