Studying differences in the course and treatment effects of psychiatric disorders between subgroups of patients can provide suggestions to improve everyday clinical practice.<br/> AIM: To illustrate how routinely registered data from child and adolescent psychiatry can be used to gain insight into differences in the development of patient groups.<br/> METHOD: Multilevel analyses in four subgroups of youths with an autism spectrum disorder (asd; n = 1681; boys/girls, with/without comorbid psychiatric disorder) to investigate differences in the development of quality of life during the first six months of treatment.<br/> RESULTS: Subgroups of youths with asd showed differences in development of quality of life, which can provide suggestions to establish personalized care.<br/> CONCLUSION: Multicenter research in large samples is needed to investigate the robustness of our findings. The 'Research Data Infrastructure', containing routine outcome monitoring and electronic medical record data from more than 117.000 youths in child and adolescent psychiatry, offers a unique opportunity to perform large scale practice based research.