The population chosen for study were, therefore, fourth-year ESO pupils from schools across the Balearic Islands. To ensure greater representativeness of the sample population, we defined diverse strata to control, in accordance with a set of variables considered to be essential for our research. These were:

1. Territorial division: the Islands.

2. Type of population agglomeration:
- towns with over 30,000 inhabitants in Mallorca and Ibiza, and 20,000 in Menorca.
- towns with less than 30,000 inhabitants in Mallorca and Ibiza, and 20,000 in Menorca.

3. Type of ownership of the school: public / private.

4. Type of secondary school in terms of the language used:
- more Catalan teaching;
- more Spanish teaching.

As "class group" was considered to be a unit of study, we used a mixed strategy when designing the sample. There were two discernible stages:

Stage One. Using data provided by the Council for Education and Culture of the Government of the Balearic Islands, the total population of fourth-year ESO pupils in schools across the Balearic Islands was calculated at 3,897 pupils in private centres and 4,939 in public schools (2000-2001 academic year).

On the basis of these criteria, we calculated that the minimum sample size had to be 384 pupils surveyed. Once we had calculated the minimum number of pupils to be surveyed, hypothesising that each class group contained 22, we calculated that the total number of class groups that had to take the survey (conglomerates, "cluster sampling") was 18 sample groups; these would be divided according to geographical situation, as shown in the following table.

We observed that 1.047% of the population were in the fourth year of ESO, and that the percentage of pupils surveyed was 4.47%.

Stage Two. At this stage, the decisions were made as to the distribution of these 18 groups on the basis of the strata. To do so, information on the variables to control was obtained and analysed.

Table 1 illustrates the sample obtained from applying the criteria discussed above, and how many surveys were obtained in each school.