III. >> M  ethodological Approaches to Human Rights Indicators >> Sources and data-generating mechanisms Censuses A census is a complete enumeration of all members of the population of a country or any other territory, unlike statistical surveys, where only selected members of the population are surveyed.27 Countries usually conduct censuses of population, housing,28 agriculture and industrial establishments. A population census is usually conducted at 10-year intervals because of the complexity and cost of the Box 19 operation. It provides basic baseline data on the structure and key characteristics of the population and on variables that do not change rapidly. The complete enumeration of the population allows variables of interest to be available at the lowest geographical level (including in principle for homeless and nomadic groups). It is a key resource for building disaggregated socioeconomic statistics as well as for generating samples for statistical surveys.29 Survey of abuse and ill-treatment by police forces in Mexico City Fundar, Centre for Analysis and Research, conducted a pilot survey to determine the extent of abuse and ill-treatment of the public by police forces in Mexico City between November 2003 and October 2004. The project used a qualitative approach to conduct in-depth narrative interviews of members of the police forces, victims of different forms of abuse, and prisoners and persons in detention centres to understand the dynamics among rights holders, the authorities and the abusers. This qualitative information provided the basis for the design of the questionnaire. Consultations on the questionnaire took place with various stakeholders and it was pretested rigorously among a number of households. The results of the pilot survey showed low levels of confidence in the police and a high incidence of abuse, particularly bribery. The survey also revealed that 94 per cent of abuse went unreported. Sources : Metagora questionnaire, 2004; Naval, Walter and Suarez de Miguel, Measuring Human Rights and Democratic Governance. 27. See glossary of statistical terms. 28. A housing census is commonly conducted along with a population census and can provide information relevant to the rights to adequate housing, water and sanitation. 29. See Principles and Recommendations for Population and Housing Censuses. Human Rights Indicators 63