The baseline investigation occurred in 1991 among 18.973 respondents in the city of Eindhoven and surrounding cities.
Two sub-samples of baseline survey respondents were invited to participate in additional in-depth interviews.
The first subsample was a random subsample.
The second subsample included an over-representation of chronically persons, based on self-reported information about chronic diseases such as coronary heart disease, diabetes mellitus, chronic obstructive pulmonary disease (COPD), or back problems.
Both samples were interviewed and/or surveyed in 1991, 1997, 2004, 2011 and 2014. In 2004 and in 2014 additional samples were drawn to amplify the study population.
The first (random) subsample was also interviewed in 1993 and 1995 and the second (chronically ill) subsample was interviewed in 1992, 1993, 1994 and 1995.
Additionally, in-depth interviews were held in 2005 among a subsample of the 2004 survey respondents, in 2012 among a subsample of the 2011 survey respondents and in 2016 among a subsample of the 2014 survey respondents.