1. Department of PharmacoEpidemiology & PharmacoEconomics; University Centre of Pharmacy; University of Groningen; Groningen The Netherlands
2. Department of Pediatrics; Medical Center Leeuwarden; Leeuwarden The Netherlands
3. Department of Epidemiology; Boston University School of Public Health; Boston MA USA
4. Boston Collaborative Drug Surveillance Program; Boston MA USA
5. Department of Epidemiology; University Medical Centre Groningen; Groningen The Netherlands