Methodological and statistical considerations for threats to internal validity in pediatric outcome data: response shift in self-report outcomes.

OBJECTIVE To examine conceptual models of response shift, research design, and internal validity issues in the context of longitudinal outcome research using self-report measures such as pediatric quality of life. METHODS Growth modeling was introduced and illustrated using a previously published data set (Clay, Wood, Frank, Hagglund, & Johnson, 1995) of adjustment in children with juvenile rheumatoid arthritis, diabetes, and healthy controls. RESULTS Demonstrations revealed that growth modeling may detect response shift and may also model the time and shape of the response shift. CONCLUSIONS Growth modeling provides one avenue to investigate response shift, thereby addressing an important threat to internal validity in longitudinal outcome research such as quality of life in children with chronic illness.