Sample
To create a sample, you must define the reference population (which we call the universe) and make a representative selection from it (which we call the sample).
The reference population for CIS studies is provided by the National Institute of Statistics (INE), using data from the population register until the end of February 2025 (study 3498) and, thereafter, data from the annual population census. The change is due to the fact that the INE has stopped producing the continuous census. These sources provide population information based on the variables of sex, municipality, nationality, and age. This data is then transformed into the strata of sex, age groups, and municipality size by autonomous community used in the surveys.
Sample design, that is, the selection of people to be surveyed, is crucial, as it allows the data obtained to be used to describe and analyze the population from which it was drawn, the universe. It is important to keep in mind that a sample has a margin of error, the magnitude of which is established in the technical sheet that accompanies each study and specifies all the technical characteristics.
Weighing
The sample is weighted by comparing the sample composition with an appropriate delineation of the sample population. This delineation of the sample population is provided by the National Institute of Statistics (INE). Based on this, a national weighting procedure is applied, using marginal and inter-cell weights. In this way, minimum variables such as sex, age, NUTS II region (basic regions defined by the Eurostat nomenclature of territorial units for statistics), educational level (using the classification of programs, qualifications, and certifications by educational levels attained (CNED-A)), and locality size are introduced into the iterative procedure. This post-stratification weighting is also known as 'correction weighting' or 'total non-response weighting'.
The CIS dataset always provides two types of weights: a post-stratification weight at the national level (PESO) and a post-stratification weight at the autonomous community level (PESOCCAA). Depending on the sample size, some studies also offer a post-stratification weight at the provincial level (PESOPROVINCIA).
The mean values of the coefficients of the post-stratification variable at the national level (PESO) are attached at the end of the technical sheets.
For the estimation at the level of each autonomous community, the microdata file also includes the weighting for each of them (variable PESOCCAA).
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