Time-constant confounders. Although they drop out of the fixed-effects models, we use the following additional variables to describe the sample: Gender (male/female); Age at baseline, categorized in age groups (<40, 41–50, 51–60, 61–70 and >70); and Socioeconomic status (SES), assessed through educational level: a variable, based on the Dutch educational system, was created with three categories: “low” (up to general secondary education), “middle” (secondary vocational education, or higher general and pre-university education), and “high” (higher professional education or university education).
