The D-score: a metric for interpreting the early development of infants and toddlers across global settings

Abstract

Introduction: Early childhood development can be described by an underlying latent construct. Global comparisons of children's development are hindered by the lack of a validated metric that is comparable across cultures and contexts, especially for children under age 3 years. We constructed and validated a new metric, the Developmental Score (D-score), using existing data from 16 longitudinal studies.

Methods: Studies had item-level developmental assessment data for children 0-48 months and longitudinal outcomes at ages >4-18 years, including measures of IQ and receptive vocabulary. Existing data from 11 low-income, middle-income and high-income countries were merged for >36 000 children. Item mapping produced 95 'equate groups' of same-skill items across 12 different assessment instruments. A statistical model was built using the Rasch model with item difficulties constrained to be equal in a subset of equate groups, linking instruments to a common scale, the D-score, a continuous metric with interval-scale properties. D-score-for-age z-scores (DAZ) were evaluated for discriminant, concurrent and predictive validity to outcomes in middle childhood to adolescence.

Results: Concurrent validity of DAZ with original instruments was strong (average r=0.71), with few exceptions. In approximately 70% of data rounds collected across studies, DAZ discriminated between children above/below cut-points for low birth weight (<2500 g) and stunting (-2 SD below median height-for-age). DAZ increased significantly with maternal education in 55% of data rounds. Predictive correlations of DAZ with outcomes obtained 2-16 years later were generally between 0.20 and 0.40. Correlations equalled or exceeded those obtained with original instruments despite using an average of 55% fewer items to estimate the D-score.

Conclusion: The D-score metric enables quantitative comparisons of early childhood development across ages and sets the stage for creating simple, low-cost, global-use instruments to facilitate valid cross-national comparisons of early childhood development.

Keywords: child development; global health; item response theory; psychometrics.

Figure 1

Distribution of the D-score by age and cohort with the final model (565 items and 18 equate groups). D-score, Developmental Score.

Figure 2

(A) Correlations of DAZ and age-adjusted original measures of early childhood development in children under 48 months with IQ and receptive vocabulary measures at Time 2 for ages >4–9 years, arranged by age at Time 1. For the original instruments, Bayley-I and Bayley-II, we used the MDI in the correlations (Bangladesh, Chile 1 and South Africa); for Bayley-III, we used the measure from the cognition domain (Colombia 1 and Colombia 2). (B) Correlations of DAZ and age-adjusted original measures of early childhood development in children under 48 months with IQ and receptive vocabulary measures at Time 2 for ages >9–18 years, arranged by age at Time 1. For the original instrument, Bayley-III, we used the measure from the cognition domain (Ethiopia). IQ measures are Denver-II, Raven’s and Raven’s (Coloured), SB-4 and SB-5, UKKI, WAIS, WISC-V, and WPPSI. Receptive language measures are the PPVT and its Spanish version, TVIP. Bayley-I, II and III, Bayley Scales for Infant and Toddler Development; Denver-II, DenverDevelopmental Screening Test; DAZ, D-score-for-age z-scores; D-Score, Developmental Score; IQ, Intelligence Quotient; MDI, Mental Development Index; PDI, Psychomotor Development Index; PPVT, Peabody Picture Vocabulary Test; Raven’s, Raven’s Progressive Matrices; SB-4 and SB-5, Stanford Binet IntelligenceScales; TVIP, Test de Vocabulario en Imagenes Peabody; UKKI, Utrechtse Korte Kleuter Intelligentietest; WAIS, Wechsler Adult Intelligence Scale - Revised; WISC-V, Wechsler Intelligence Scale for Children; WPPSI, Wechsler Preschool and Primary Scale of Intelligence