91 Patient cost-sharing, mental health care and inequalities APPENDIX C. STUDY POPULATION AND OBSERVATIONS The study population corresponds to individuals born between 1990 and 1997 and living in the Netherlands during 2009. It consists of 1,541,210 individuals, after excluding 2.9% of the individuals due to missing health care expenditures for one or more study years, and an additional 1.4% due to missing income or demographic controls. The matrix in Figure A1 shows how many person-years correspond to each individual in the study population, which includes those turning 17, 18 and 19 years old during the study period. The number of grey cells per row shows that the study population includes 1 person-year per individual for birth cohorts 1990 and 1997, 2 person-years for 1991 and 1996, and 3 person-years for 1992-1995. Figure A1. Birth cohorts and years used to define study population. Notes: Observations identified with 17 in a certain year correspond to individuals with their 17th birthday in that year (204 to 215 months of age at December 31st ). Analogously, 18 identifies individuals between 216 and 227 months and 19 between 228 and 239 months. In the final estimation sample we further exclude observations of the calendar year turning 18 (except for those born in December – see motivation provided in 3.2) and all observations in the year 2012 (see motivation provided in appendix A). These exclusions result in a final population of 2,004,477 person-years. 3
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