Instability of performance on cognitive tests in infants and young children with Down's syndrome.

Stability of cognitive performance in assessment situations was investigated in a group of 18 children with Down's Syndrome (DS) aged 6 months to 4 years. Two widely used tests of early cognitive development were presented: the Mental Scale of the Bayley Scales of Infant Development and a series of Piagetian object concept tasks. Both sets of tasks were presented in the same testing session, with testing repeated one to two weeks later. Both qualitative and quantitative aspects of performance were recorded. Even when overall scores were similar, patterns of pass/fails were very different in the two testing sessions. Successes showed poor stability over sessions with fails often occurring by default, the result of a refusal to engage fully in a particular task. This pattern of results suggests: (1) that single-session testing may not adequately assess cognitive status in DS children; (2) that the relationship between performance and competence may be unstable in such children, and (3) that "slow development" theories may not adequately describe cognitive development in young children with DS.