Half of each sample from 1981 and onwards have been drawn from the gross sample that was used in the prior survey - i.e. a rotating panel. To make sure the panel data is reprsentative, it has been supplemented with voters in the age group 18-21 because they did not have the right to vote in the previous survey. From 1997 and onwards, the panel was also supplemented with people who immigrated to Norway between the surveys.
