Background: The Nisonger Child Behavior Rating Form (NCBRF) allows for the assessment of frequency and severity of challenging behaviours of individuals with ID with one combined rating. However, frequency and severity rating of challenging behaviours may be in disagreement. Therefore, the efficacy of frequency and severity independent ratings for each NCBRF item was investigated. Method: Professional caregivers of 105 individuals with ID completed the NCBRF with separate frequency and severity ratings and the Vineland-II. Results: Magnitudes of correlation coefficients between frequency and severity rating of each NCBRF item were mostly trivial, small, or medium. However, the differences between the magnitude of the corresponding correlation coefficients of frequency and severity rating of each NCBRF item when related to the Vineland-II adaptive behaviour dimensions were rarely statistically significantly. Conclusions: Despite the need for further comparisons with other challenging behaviour scales, the separate use of frequency and severity ratings seems to be useful.