Assessment of children' Quality of life (QOL) is a special challenge for clinicians and researchers because different cognitive abilities of children at various ages and illness levels are so varied. In addition, statistical strategies reported to evaluate proxy agreement have been inconclusive. The specific aims of this study were to examine agreement between child self-reports and parent proxy-reports to evaluate QOL in a sample of pediatric cancer patients. Previously tested QOL instruments (Quality of Life for Children with Cancer, QOLCC) were completed by 141 patients (82 children and 59 adolescents) and 141 of their parents. Three different statistical approaches were employed to evaluate convergence of self-report and proxy-report: product-moment correction coefficient, intraclass correlation (ICC), and comparison of group means. In addition, scatter bias was used to examine the degree of differences across the range of measurement. Our findings indicate that neither Pearson product correlation, ICC or group difference provided enough information to detect the individual differences of measures of QOL. We found that scatter bias should be supplemented to quantify the degree of individual-level differences. The results suggest that when children who are younger are not able to evaluate QOL assessment due to their developmental limitation or severity of illness, parents can provide valid information about their QOL. However, parent-proxy of QOL for adolescents provides significantly different information than self-report and proxy data of QOL for adolescents should be used with caution.

The predictors of agreement between patients' reports and parent-proxy reports, including the passage of time (from baseline to 6-month follow up), gender, age, and illness severity, have not been conclusively determined. Further studies are needed to examine patient-proxy agreement during a longer follow up period at more than two points in time and using the same patients through the stages of confirmation of diagnosis, acceptance of diagnosis, treatment, and after treatment.