Internal Consistency and Test–Retest Reliability
The internal consistency was assessed separately in the ASD group and in the comparison cases. Cronbach’s coefficient alpha for the total scale was estimated at 0.92 in the ASD group and at 0.94 in the comparison cases. Internal consistencies for the four subscales were: social interaction α = 0.87/0.89 (ASD group/comparison cases), language, α = 0.58/0.22 (ASD group/comparison cases), circumscribed interests α = 0.73/0.73 (ASD group/comparison cases), and sensory motor α = 0.81/0.77(ASD group/comparison cases). Item 2 in the language subscale (I often use words and phrases from movies and television in conversations) had a negative corrected item-total correlation, and by removing it alpha for this subscale could be increased to 0.70/0.40 (ASD group/comparison cases).
Test -retest reliability was assessed in a subset of subjects comprising 12 with ASD who had completed RAADS-R on two separate occasions with 3–6 months interval. The total scores on the two occasions were strongly and positively correlated (r = 0.80, p = 0.002). Strong and significant correlations were also obtained for three of the subscale scores: social interaction (r = 0.76, p = 0.004), circumscribed interests (r = 0.73, p = 0.002) and sensory motor (r = 0.84, p = 0.001). For the language subscale the correlation was not statistically significant (r = 0.43, p = 0.161).
Correlation with the Autism-Spectrum Quotient
The degree of agreement between RAADS-R and AQ was assessed by comparing 35 subjects with ASD and 49 comparison cases. Correlation analyses between RAADS-R and AQ total and subscale scores were performed separately in the comparison cases and in the ASD subjects. In the ASD group there was a strong, positive correlation between RAADS-R and AQ (see Table ).
Correlations (Pearsons r) between RAADS-R and AQ total and domain scores in 35 subjects with ASD
In the comparison cases, a Spearman’s rank order coefficient was computed as the variables were not normally distributed. The correlation between AQ and RAADS-R total scores was strong (ρ = 0.70, p < 0.0001). RAADS-R subscale; social interaction, circumscribed interests, and sensory motor all had moderate to strong correlations with AQ Total score (ρ = 0.51–0.72, all p < 0.0001). The language subscale however was not significantly correlated with AQ total or any of the subscale scores (ρ = 0.06–0.17, all p > 0.05).
Distribution of Scores
The distribution of scores is shown in Fig. a–e. As evident from the histograms, the scores of the comparison cases had strong positive skewness (2.17) and were markedly peaked (kurtosis = 6.78). The scores of the ASD group did not depart significantly from normality (skewness = 0.02, kurtosis = −0.33). The median, minimum, and maximum scores of the two groups are shown in Table .
Total RAADS-R and domain scores in the comparison cases group and ASD group. a Total RAADS-R. b Subscale: Social interaction. c Subscale: Language. d Subscale: Circumscribed interests. e Subscale: Sensory motor
Median, minimum and maximum RAADS-R total and domain scores in the ASD group (N = 75) and the Comparison cases (N = 197)
Group and Sex Differences
To explore group differences ANOVAs were performed comparing RAADS-R total and subscale scores by Diagnosis (ASD versus comparison cases) and Sex. Five subjects were excluded from the analysis due to missing information on sex. Mean total and subscale RAADS-R scores for the ASD and comparison cases are shown in Table , together with the results of the ANOVAs. As indicated, main effects of Diagnosis were found across all tests, the ASD subjects scoring higher than the comparison cases on the full scale as well as all four subscales. There was no main effect of Sex on the Total score, but there was a significant two-way interaction between Diagnosis and Sex. In the comparison cases the males obtained higher total scores than females, whereas in the ASD group females obtained higher total scores than males. T-tests revealed that these sex differences were not significant when assessed either in the comparison cases (t = 1.193, df = 137, p = 0.166) or in the ASD subjects (t = −1.847, df = 69, p = 0.069).
Mean RAADS-R total and domain scores and standard deviations for males and females in the comparison cases versus ASD group
On the social interaction and circumscribed interests subscales there were no significant main effects of Sex, nor any Diagnosis × Sex interaction. On the language subscale there was no main effect of Sex, but a significant Diagnosis × Sex interaction, comparison case males scored higher than comparison case females (t = 2.370, df = 194, p = 0.019). Females in the ASD group scored somewhat higher than males, though this difference did not reach significance (t = −1.801, df = 69, p = 0.076). Lastly, on the sensory motor subscale women generally obtained higher scores than men. There was also a significant Diagnosis × Sex interaction. T tests showed that women in the ASD group, but not in the comparison cases, obtained significantly higher scores than males (t = −3,769, df = 69, p < 0.0001).
Eight comparison cases and 3 subjects in the ASD group had scores that deviated markedly from the mean (at least 2 standard deviations). If these outliers were excluded from the analysis, the patterns of which Diagnosis and Sex differences were significant did not change.
Ability to Differentiate Between ASD and Comparison Subjects
In order to further examine the ability of RAADS-R to distinguish between the two groups, a ROC-graph was generated. The area under the ROC curve (AUC) was estimated at 0.96 (Std. err. 0.012, 95% CI 0.94–0.98), indicating high overall accuracy. This means that the probability of a randomly selected subject with ASD scoring higher than a randomly selected subject with no ASD was approximately 96% in this sample. Table shows the sensitivity and specificity of RAADS-R total score at various cut-offs between 50 and 100. If sensitivity and specificity are given equal priority, a cut-off of 72 achieved the best compromise, with sensitivity 0.907 and specificity 0.929.
Sensitivity and specificity of RAADS-R at various cut-off scores (N = 272)