Official statistics are one of the main sources of information about our society. They are essential for public administrations, companies and other social agents to be able to make better decisions. They also serve to provide society with information for the analysis and evaluation of government policies. Other statistics have legal effects, for example to update incomes.

An example of the use of statistical information is the calculation of Gross Domestic Product (GDP). To calculate GDP, data from different surveys are used (Labour Force Survey, Household Budget Survey, Industrial Company Surveys, Annual Services Survey, Construction Industry Structure Survey, etc.). Its value determines, among other points, what our country's contribution to the Community budget should be. It is also used to calculate the participation of the Autonomous Communities in the Inter-territorial Compensation Fund, as well as to determine those that may be recipients of certain structural funds.

There are two ways of obtaining information from a population: through a census or a survey. In a census, all the units that make up the population are investigated. Surveys, however, allow information to be obtained from a group by investigating only a part, known as 'the sample', which represents the entire population.

Statistical theory ensures that valid survey results are obtained within confidence intervals. A fundamental condition for this is to obtain the cooperation of all the units in the sample.

The usual process for sample selection depends on whether it is a household or an enterprise survey.

In the case of household surveys , each municipality is made up of geographical areas called census tracts. There are 36,000 census sections in total. Census sections are grouped according to socio-demographic characteristics. In a first stage a sample of census sections and, in a second stage, within each section a sample of dwellingsis drawn. Depending on the survey, information is requested from all the residents of the dwelling or from a sample of them. The number of dwellings in the sample depends on the degree of detail of the estimates provided by the sample. Thus, the more sporadic the characteristics under investigation or the greater the geographical breakdown sought, the larger the sample size should be.

In the surveys addressed to enterprises, the sample collection phases are as follows:

• Grouping of enterprises or local units that have similar characteristics. The groups are called strata. The geographical area of location, economic activity and a measure of economic size, usually the number of employees, are usually taken into account.
• Determination of the enterprises or local units required in the sample within each stratum.
• Selection within each stratum of the sample units. In this last stage, if the sample is a part of the total population, the selection is made randomly. There are circumstances that require all units in the stratum to be part of the sample. This is the case for larger units or if there are few enterprises in the stratum.

A more detailed description of the selection process can be found in the introduction to sample design section. In addition, more information on the characteristics of the samples can be found in the methodology of each survey.

The information collected by the INE in its surveys is confidential, and is protected by statistical secrecy. The INE does not cede any individual data to other administrations. There is one exception, which is the transfer to other bodies that compile statistics (e.g. Autonomous Community or European Institute), provided that they offer the same level of data protection. This is done so that respondents do not have to provide the same information twice. Further information can be found in the confidentiality section.

If the survey is included in the National Statistical Plan, the Public Statistical Function Act and its subsequent legislative development determine that the survey is compulsory. You can find out whether the survey is compulsory on the survey page, on the questionnaire (if it is, it will be explicitly mentioned) or by asking the interviewer.

For more information, please see the section on mandatory surveys.

On the INE's home page, you will be able to access the data obtained from all the research carried out, free of charge, just like the rest of the citizens.

In addition, the informants of the INE surveys will receive a discount on INE publications as well as on customized requests for the information it produces. You may learn about the discounts that our survey informants benefit from by accessing the Price Resolution.

In any case, their collaboration is very important in order to have reliable statistics that allow us to take the most appropriate decisions for a better economic development, from which all Spaniards will benefit.

The INE uses information from administrative records of other public administrations when possible, but these do not always contain the information investigated by the INE, so it has to be completed with sample surveys where the reporting units must collaborate.

You should receive an official letter from INE informing you that you have been selected. The letter will include a 900 number that will allow you to contact them.

You may also contact the network of Provincial Delegations of the National Statistics Institute.

In the case of household surveys that are investigated through interviewer visits, interviewers must identify themselves by showing the card that identifies them as personnel authorised to conduct interviews for the INE.