The validation of parallel test forms: 'Mountain' and 'beach' picture series for assessment of language skills

Pictures are widely used to elicit expressive language skills, and pictures must be established as parallel before changes in ability can be demonstrated by assessment using pictures prompts. Why parallel prompts are required and what it is necessary to do to ensure that prompts are in fact parallel is not widely known. To date, evidence of equivalence has been documented for only a handful of picture instruments. The present study aims to demonstrate that two picture series, one describing hiking on a mountain, the other a picnic at a beach, designed to elicit narrative writing from children, are statistically parallel. To verify their being parallel, a random group design and a covariate were used. MANCOVA and confirmatory factor analysis confirmed that the two alternate picture forms were parallel in terms of means, variances, and factor structures with regard to multiple writing components. This study makes a unique contribution to picture-based assessment, first by publishing this parallel picture set and making it available as a tool for assessing children's productive skills using longitudinal designs, and second by demonstrating a process by which any other test forms, pictorial or otherwise, can be validated as parallel.