Baby Observational Selective Control AppRaisal (BabyOSCAR): Construct validity and test performance

Aim: To investigate the construct validity of the Baby Observational Selective Control AppRaisal (BabyOSCAR), an assessment of independent joint motion in infants with cerebral palsy (CP). Method: BabyOSCAR was scored for 75 infants (45 with CP and 30 without CP). Rasch analysis was used in combination with classical test theory to assess areas of strength or improvement. Overall fit and precision, unidimensionality, local independence, reliability indices, Wright's child-item map, and differential item functioning were examined as part of Rasch analysis to investigate the item properties, internal construct validity, and reliability of BabyOSCAR. Cronbach's α was used to evaluate items’ internal consistency. Results: Analysis demonstrated good fit to the Rasch model, with only one erratic item. Unidimensionality results suggest two dimensions, split between arm and leg items. Item calibration reliability was between 0.84 and 0.86, with three distinct item difficulty levels. Infant measure reliability was between 0.82 and 0.91, separating infants into three ability levels. Together, the two subscales covered the full range of skills, with redundancy mostly between the same motion on both sides of the body. Cronbach's α was between 0.90 and 0.95. Interpretation: BabyOSCAR's construct validity was supported. Arm and leg subscales can be translated to a logit scale. What this paper adds: Baby Observational Selective Control AppRaisal (BabyOSCAR) has excellent construct validity with good overall fit and precision. Individual BabyOSCAR items contribute and work well together, forming an interval-level assessment. BabyOSCAR has two separate subscales, arms and legs, that complement each other. BabyOSCAR's items represent a continuum of skills with three distinct difficulty levels. BabyOSCAR's continuum of skills reliably separates infants into three ability levels.