4.1. Universe
The Universe consists of individuals, of both sexes, aged 15 or above, resident in Portugal (Continent and Autonomous Regions of Azores and Madeira).
4.2. Sample
The sample consists of 1,080 individuals, stratified by region (Greater Lisbon, Greater Oporto, Autonomous Regions of Azores and Madeira and Rest of the Country) and by habitat (population size of locality). Each stratum considered, resulting from the application of these two variables, was assigned a number of interviews in proportion to its weight in the Universe.
The sample has a confidence level of 95%, with a sampling error of 3%. Based on the 1991 Census (INE), the sample distribution, by regions and strata, was as follows:
Table 1 - Distribution of sample by region
| REGIONS | Universe (103) | % in universe | sample | % in sample |
| Greater Lisbon | 1.514 | 19.8 | 243 | 22.5 |
| Greater Oporto | 934 | 12.2 | 156 | 14.4 |
| Autonomous Regions | 365 | 4.8 | 71 | 6.6 |
| Rest of Country | 4.826 | 63.2 | 610 | 56.5 |
| Total | 7.639 | 100.0 | 1080 | 100.0 |
Table 2 - Distribution of sample by population strata
| POPULATION STRATA | Universe (103) | % in universe | sample | % in sample |
| -2.000 inhabitants | 3.788 | 49.6 | 285 | 26.4 |
| 2.000/5.000 inhabitants | 698 | 9.1 | 113 | 10.4 |
| 5.000/10.000 inhabitants | 485 | 6.3 | 121 | 11.2 |
| +10.000 inhabitants | 1.703 | 22.3 | 310 | 28.7 |
| City of Lisbon | 663 | 8.7 | 155 | 14.4 |
| City of Porto | 302 | 4.0 | 96 | 8.9 |
| Total | 7.639 | 100.0 | 1080 | 100.0 |
The locations were selected at random through a system of systematic sampling, with a random starting point, with exception of the cities of Lisbon and Oporto.
In order to attribute an equal number of interviews to each locality, the total assigned to the habitat/population stratum was divided by the population of each locality selected.
4.3. Information collection technique
The technique used was direct, personal and personalised interviews, with a questionnaire prepared by a specialist company, in collaboration with the Instituto das Comunicações de Portugal.
The questionnaire was subject to a pre-test (pilot survey) before the start of the fieldwork in order to confirm its operability.
As a back-up, all interviewers were provided with an instruction manual.
4.4. Collection of information
Fieldwork took place between March 29 and April 12, 2001.
