ABSTRACT
Patient-reported outcomes (PROs) measures are often incorporated into a study by administering questionnaires at multiple time points with the goal of characterizing the outcome over time (Fairclough, 2004, 2005, 2010). These measurements are provided by the same individual repeatedly over time, allowing for the direct study of change. Such longitudinal data arise in most PRO investigations because interest centers on how a disease or intervention affects an individual’s functioning and well-being over time. Longitudinal analysis also considers how groups change over time and how between-group factors like treatment affect different intervention groups over time. Longitudinal analysis has become central in drawing inferences about PRO measures.
