This study examines the external validity of a nationwide dataset, considering the fact that clients can refuse the extraction of their clinical information to aggregate datasets. Data of 949 youths (56% boys, mean age 15.6 years, SD = 1.49) from six Dutch residential institutions were included. An individual-variables and a person-oriented approach were applied to examine the extent to which the group giving permission to the use of their data for aggregate purposes (66%) is representative for the total population in secure residential youth care. Both static, non-changeable client information as well as dynamic, changeable client information were gathered. Youths who assented to the use of their information were overrepresented amongst the population of some of the participating institutions, and were more often Caucasian. Three start profiles based on the dynamic variables were distinguished. Youths who assent were overrepresented in the most problematic profile, whereas youth who did not assent were overrepresented in the least problematic profile. To improve the external validity and the value of the monitor, it is important to achieve more uniformity between institutions as far as the procedures are concerned, and to find creative ways to increase the assent levels, particularly among ethnic minorities.