Follow-up and census data

Date of death, emigration and cause of death until 31 December 2016 were obtained from the National Cause of Death Registry. All inhabitants of Norway have a unique personal identification number that allows complete follow-up until death or emigration. In total, 156,896 persons were initially observed. A total of 103,881 respondents with known smoking status and complete responses were followed; median follow-up was 27.4 years, maximum 45.2 years. They represented 2,449,538 person-years at risk.

Highest attained education was extracted from the national census for each decade and grouped according to the maximum length of education using three levels: compulsory education (7–10 years), medium level (11–13 years) and university level (≥14 years).

The overall inclusion rate in the analyses was 65% of the target population and varied for the baseline examination from 81% of the target population of the cohort of 1972 to 62% in the cohort of 1998–1999. The distribution of participants of the various groups was almost identical in the target population and the population of respondents with regard to age, sex and education (S2 Table).