In the metropolitan area the primary units are stratified according to percentage of workers. Selection of persons. All members aged 15 and more in the households selected are listed in fixed order on selection cards. The interviews were carried out by approx. For each assignment the interviwers were given a list of the addresses at which interviews were to be made as well as written interviewer instruction concerning the questionnaire.
Weighting is carried out concerning degree of urbanization, sex and age.
However, in some cases supplementary variables are used when considered necessary. In addition, an internal control is made of the quality of the answers, of whether the time limits set for the interviewing are kept, whether all the questions have been asked, etc. This control is not reflected in the data material.