We can distinguish between two types of population statistics included within the ordinary statistical output of Idescat and the Catalan statistical system, depending on where the data come from:
Population estimates and projections are operations conducted based on the information from the latest census, among other sources, for this reason they are called census-based operations.
For the recent past, every year we have the municipal population register and the population estimates. For the present and future we only have the population projections.
The municipal population register is the administrative record containing the residents of a given municipality and it constitutes proof that they reside in the municipality and have their usual home there. The exploitation of the municipal population register provides figures on the number of people registered in the population census in each municipality in Catalonia. Spanish laws on the local regime sets the rules for carrying out the municipal population register, which is the responsibility of the town halls, and for the data gathering, which must be revised on the 1st of January every year, under the supervision and coordination of the INE. These figures are declared official by the Spanish government in order to determine the number of deputies per election district, take decisions that affect the financing and competencies of the municipalities, and other purposes provided for by law. The official figures are available by municipality and they are broken down according to sex, age, nationality and place of birth. During the second quarter of the year, provisional data is published that is considered valid until the official data is made available (see Municipal census). The provisional data is only available for Catalonia and the provinces.
Population censuses are statistical operations conducted by INE in conjunction with Idescat in the different phases. They include an exhaustive investigation of the residents of the entire country and detailed questionnaires on their demographic, social and economic characteristics. They provide extremely detailed regional and thematic information on the level of municipality, district and census section. They are conducted every ten years, and the last one was conducted with the reference date 1st of November 2011.
These are census-based statistical operations whose goal is to estimate the current size of the population living in Catalonia or to project the future population using the best information available at any given time on the evolution of the demographic phenomena that determine the population. Depending on the time period they cover, we can distinguish between population projections, postcensal population estimates and intercensal population estimates.
Population projections are a forecast of the future population calculated based on the starting population which is normally gotten from the latest population census, and using a series of hypotheses about the future developments in birth rate, death rate and migrations. The population projections are available by county and for the municipality of Barcelona, and they are broken down by sex and age.
Postcensal population estimates are a kind of synthesis statistics whose goal is to establish for any given point in time the population series living in Catalonia at the level of region, county and municipalities of 50,000 inhabitants or more, broken down by sex and age. When drawing up these statistics, the best and most up-to-date information available is used, including the latest population census, and the goal is to apply the most relevant demographic estimate techniques. The figures enable us to reliably calculate demographic, social and economic indicators in which the population figures are the denominator of reference. During the second quarter of the year, preview data is published and is valid until it is replaced by the definitive postcensal estimates. The preview data is only available for Catalonia, the provinces and Areas of the Territorial Plan.
Once the information from the latest population census is available and the demographic evolution between this census and the previous one is known, this information is used to revise the population estimates for the intercensal period. Intercensal estimates provide us with an annual series from the 1st of January 1981 until the 1st of January 2012, which is totally internally coherent and is later linked with the postcensal population estimates and the population projections. The regional and thematic breakdown of this information is the same as for the postcensal estimates.
This is a new statistical operation developed for the first time in 2007 by Idescat and whose central objective is to update every three-four years the information on the main structural variables referring to the population, households and families and housing. The Demographic Survey enables us to keep the classical variables on the demographic structure up-to-date, such as marital status, family structure, migrations, educational level, knowledge of knowledge, relationship with the economic activity and housing. This provides partial continuity with the historical series derived from the population censuses, the revisions of the municipal censuses and the 1996 population statistics. The population figure in the Demographic survey corresponds to the postcensal population estimates calculated on the date of reference of the survey, November 1 of the year of each edition. Therefore, it is a census-based source.
These sources are used for different purposes. The municipal population register is a source for administrative purposes, and its figures are declared official by the government of Spain. The population census and population estimates are statistical sources whose purpose is to set up the series on the population residing in Catalonia at any given time. The figures obtained from the municipal population register are systematically superior to the ones from the population estimates, partly because the municipal population register does not exhaustively compile departures abroad. Administrative corrections, such as withdrawals because of the expiration of non-EU nationals without permanent residence permits, are concentrated into certain years and mean that the municipal population censuses show leaps that correspond not to actual population movements but to the past.
The coexistence of different population figures for the same reference date is a widespread situation in the realm of international official statistics, with systems based censuses, estimates and projections with different timeframes. What is characteristic of the population statistics in Spain and Catalonia is the existence of another type of figure coming from the statistical exploitation of the municipal population register. Only countries with highly evolved population records, such as the Nordic countries, can base their demographic statistical system on these records, yet they nonetheless still need projections on the nearer or more distant future.
There are two types of population figures included within the output of the INE and the national statistical system for Spain:
Basically, this is the same situation as in the Catalan statistical system.