Convergence of Mullen Scales of Early Learning Developmental Quotient with the Differential Ability Scales, Second Edition Intelligence Quotient in Young Children

Background:  

Cognitive test selection is a recurrent challenge in autism spectrum disorder (ASD) research. Emphasis of common IQ tests on verbal ability, even on the so-called nonverbal testlets, can cause “floor” effects when standardized scores are not available for a given child’s age range and raw score. Faced with these challenges, researchers often report the developmental quotient (DQ; mental age ÷ chronological age) in lieu of a standard score. Researchers also often choose to report only nonverbal DQ or IQ, with the understanding that these scores represent a more pure estimate of cognitive ability in individuals with ASD and significant verbal deficits. Due to these limitations in other tests, increasingly, the Mullen Scales of Early Learning (MSEL) and the Differential Ability Scales, Second Edition (DAS-II) have been used interchangeably, as both IQ and DQ, in cross-sectional and longitudinal analyses. Bishop et al. (2011) found that the MSEL had good convergent validity with the original DAS, but no data have been published on the relationship of the MSEL to the DAS-II, which has several notable changes from the first edition. 

Objectives:  

This study replicates and extends the Bishop et al. (2011) MSEL and DAS convergent validity analyses using the nonverbal (NV) and verbal (V) subscales of the MSEL and DAS-II in children with ASD and developmental delay (DD) versus typically developing (TD) children. Of primary interest was the MSEL DQ versus the DAS-II IQ, which is a relevant comparison for situations in which a child’s chronological age exceeds his or her developmental level. 

Methods:  

Participants were drawn from a larger, longitudinal study of autism. Participants were 112 children aged 2 years to 9 years (M±SD=5.4±1.3), with a diagnosis of ASD or DD (n=70; 66% ASD) or TD (n=42). Mean scores in the TD group were DAS-II VIQ=113.9±11.7, DAS-II NVIQ=116.2±10.2, MSEL VDQ=105.9±10.8, MSEL NVDQ=106.0±11.8. Mean scores in the ASD+DD group were DAS-II VIQ=71.2±22.6, DAS-II NVIQ=77.9±20.2, MSEL VDQ=58.2±18.1, MSEL NVDQ=66.5±18.6. 

Results:  

MSEL NVDQ and DAS-II NVIQ were significantly (p<.05) correlated in both groups (r_ASD+DD=.86, r_TD=.45). However, the correlation between MSEL VDQ and DAS VIQ reached significance only in the ASD+DD group (r_ASD+DD=.90, p<.001; r_TD=.30, p=.051). For both nonverbal and verbal subscales, the interaction between group and MSEL in predicting DAS-II was significant, indicating that the relationships differed significantly between groups. Of further interest was the intercept in this model, which indicated that the DAS-II was an average of 20 points higher than the MSEL regardless of MSEL score. The groups did not differ significantly on the proportion with a MSEL/DAS difference of greater than one standard deviation for nonverbal (n_ASD+DD=21, 31%; n_TD=14, 33%) or verbal (n_ASD+DD=33, 48%; n_TD=14, 33%). 

Conclusions:

This study finds support for the concurrent use of MSEL DQ and DAS-II IQ scores in ASD samples that span the range of cognitive and language abilities. However, caution is warranted when more cognitively able children are tested, especially using the verbal subscale, as the relationship between tests was much weaker when scores were higher.