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Chapter 362MethodStudy designFor this retrospective database study, we used databases from the Netherlands Institute for Health Services Research Primary Care Database (NIVEL-PCD) 12 and Statistic Netherlands (SN). 13 NIVEL collects healthcare data from routine electronic health register systems from over 400 representative Dutch general practices, covering approximately 10% of the Dutch population.12 The NIVEL-PCD files provided data regarding personal characteristics, type of health problems, consultations, and medication prescriptions from 1-1-2015 to 31-12-2018. MH problems were identified by ICPC-P codes (International Classification of Primary Care, Psychological category). 14 Medication prescriptions were coded according to the Anatomical Therapeutic Chemical (ATC) classification system up to level 3, the pharmacological subgroup. 15 We included persons 18 years or older in 2015 who remained in the NIVEL-PCD for > 1 year. Selection of study objectivesTo identify people with MID in the NIVEL-PCD, that database was linked with an SN-MID database. In this database, persons are identified who, in 2015, used services under the Dutch Chronic Care Act, the Disability Benefit Act, or the Sheltered Employment Act with an MID registration (Figure 1).The comparison group without ID was not shown with an MID in the SN-MID database, had no ICPC code P85 (mental retardation) in the NIVEL-PCD, and was not eligible for ID-specific care according to the SN-Dutch Chronic Care Act database. We matched each individual from the MID group by age, sex, and number of years in the dataset to four random individuals in the group without ID, to allow a robust comparison without overpowering.We compiled an MID-MH group containing people who had one or more MH problems coded with an ICPC-P other than P85. An additional matched control group was compiled from the group without ID.This study was reviewed and approved by the Medical Ethical Committee of Radboud University Medical Center (2017-3921) and conducted according Katrien Pouls sHL.indd 62 24-06-2024 16:26