The 1970 census questionnaires were completed by a mailout/mailback procedure. which was developed in preference to a doortodoor enumeration system used in past census taking. Yet the possibility of an undercount remains high since a number of areas in the Southwest have Spanishspeaking persons who live in substandard housing to which mail is not delivered. Migrant workers. of which a significant portion are MexicanAmericans. do not generally reside in any one location long enough to establish a mailing address. Similarly. mail delivery is often poor in inner city barrios. These places would tend to be missed by the address registers.