Objective: The Attention-Deficit/Hyperactivity Disorder Rating Scale-IV (ADHD-RS-IV) assesses ADHD symptoms in children and adolescents. The original United States norms comprise percentiles. Yet, no Nordic percentile norms exist, and only T-scores, which (often falsely) assume normally distributed data, are currently available. Here, we for the first time provide Danish percentile norms for children aged 6-9 based on parent/caregiver-reports, and illustrate the potential consequences of T-scores when derived based on the expected skewed distribution of an ADHD scale in the population.
Materials and methods: The sample comprised 1895 Danish schoolchildren (879 girls and 1016 boys) in 1st, 2nd, or 3rd grade from the general population. Their parents/caregivers completed the ADHD-RS-IV. Sex and age differences were investigated, percentiles were derived based on the observed score distributions, and for comparison, T-scores > 70 were estimated, which are expected to identify the top 2.3% under the assumption of normality.
Results: Boys were rated to have higher ADHD-RS-IV scores than girls except on the impulsivity score. No age effects were found on the majority of scores. Sex-stratified and unisex percentiles (80, 90, 93, 98) were reported. The distribution of ADHD-RS-IV scores were highly skewed. T-score cutoffs identified a significantly higher proportion of and about twice as many children as having elevated ADHD symptoms than expected (4.3-5.2% vs. 2.3%).
Conclusions: ADHD-RS-IV (parent/caregiver-report) percentile norms for young Danish schoolchildren are now available for future reference. The use of percentiles is considered appropriate given the skewed score distribution and since T-scores appear to over-identify children as having clinically elevated ADHD symptoms.
Keywords: ADHD Rating Scale IV; ADHD-RS; Percentiles; Schoolchildren; T-scores.