Objective
The objective was to describe observed differences between the official Danish Strengths and Difficulties Questionnaire (SDQ)-norms and data from a sample based on the Danish National Birth Cohort (DNBC) including children born between 1996 and 2003. We compared the risk classification, cut-off bandings and the group sizes between the Danish norms and the values found in our sample at ages 7, 11 and 18 years.

Results
Two sets of norms are used in Denmark: Arnfred’s norms, based on a sample from one single Danish municipality and Niclasen’s norms, based on multiple Danish cohorts, including the DNBC. Inconsistencies were found between banding scores in the two existing norms and the banding scores identified in our sample from DNBC: discrepancies included banding scores for several of the problem scales for children and preadolescents. For adolescents, we found less apparent inconsistencies between Arnfred’s sample and the DNBC. Results demonstrate that the existing SDQ norms do not apply well to a large-scale cohort sample in Denmark. The usefulness of the SDQ as a screening instrument for mental health problems depends on appropriate norms. We therefore urge that the current Danish SDQ norms are used with caution, and preferably they should be revised.