A number of well-validated quality of life (QOL) measures for adults have been developed, many of which are used in routine clinical trials. The inclusion of QOL measures in clinical trials has in part come from increasing recognition that self-reports on subjective states can provide information about the consequences of treatment plans (such as behavioral or psychological outcomes) that may not be captured by traditional outcome indices . In the last twenty five years, a number of well-validated child instruments have been developed .
Given the lower cognitive and language skills of young children, the majority of child QOL instruments have been developed for children above eight years with proxy reports (usually parent) used to gain information about younger children . However the value of obtaining children's self-reports about their health, functioning, abilities, and emotions is increasingly recognized within both medical care and child health research . Several generic and disease-specific QOL measures are now available that include parallel child and parent proxy-report versions (for example, generic: the Pediatric Quality of Life Inventory™ (PedsQL™) [4, 5], the Child Health and Illness Profile – Child Edition (CHIP-CE™) , and the KINDL™ , disease-specific: the Cystic Fibrosis Questionnaire (CFQ) , the Child Health Ratings Inventory (CHRIs) , and the How Are You? (HAY) ).
The availability of measures with parallel child and parent versions has raised questions about the level of agreement between children's own views and those of their parents about child functioning. The literature is relatively confused, with reported of poor parent-child agreement [e.g., [11, 12]], and of moderate to high agreement [e.g., [13, 14]]. Parent-child agreement may be affected by a number of variables . In a review of the relationship between child and parent QOL ratings, Eiser and Morse  concluded agreement is dependent on the domain being measured, with higher agreement for physical aspects of health compared to emotional or social aspects. Eiser and Morse  also reported evidence of higher agreement between parents and chronically sick children compared with parents and healthy children. Some researchers have found evidence that parents of sick children tend to underestimate their child's QOL compared with children's own ratings [e.g., ]. The reverse (i.e., overestimation) has been reported with parents of healthy children [e.g., [13, 18, 19]].
Agreement between child and parent proxy-ratings may also vary by the age of the child. Eiser and Morse  identified only two studies examining the effect of age [4, 13]. Varni et al.  reported that agreement was highest between children with cancer and their parents for cognitive functioning, and highest between adolescents and parents for physical functioning. Theunissen, Vogels, Koopman, Verrips, Zwinderman, and Verloove-Vanhorick  found that parent-child agreement was related to child's age and their positive emotions ratings. Specifically, Theunissen et al.  reported that older children (10–11 years) with low positive emotion scores agreed less with their parents than younger children (8–9 years), and older children with high positive emotion scores agreed more with their parents. A study by Annett, Bender, DuHamel, and Lapidus  with children with asthma reported parent-child agreement increased with child age. Ronen, Streiner, and Rosenbaum  reached similar conclusions, with younger age predicting greater differences between parents and children with epilepsy.
An additional factor for consideration here is the impact of parents' own functioning and well-being. Eiser, Eiser, and Stride  found that mothers who rated their own well-being as poor also rated their child's QOL as poor, suggesting that parents project their own feelings on to judgments about the child's functioning. In addition, Goldbeck and Melches  reported a significant interaction effect of parental QOL and patients' self-reported QOL in predicting parental proxy reports of their children's QOL.
Part of the confusion described above may relate to the statistical methods employed to compute parent-child agreement. The most frequently used statistic for examining agreement between child and parent reports has been the Pearson product-moment correlation coefficient . However Pearson r values provide information on the covariation among scores but do not indicate absolute agreement . A more appropriate statistic for examining agreement between raters is the Intra-class correlation coefficient (ICC). ICC values provide an index that reflects the ratio between subject variability and total variability .
It is useful to examine mean differences between children's and parents' reports, as it is possible for their scores to be correlated (i.e., linearly related) but also show statistically significant differences in mean scores . Analyses which include both correlation and mean difference testing are needed in order to provide more conclusive evidence regarding the relationship between parent and child ratings. We identified two studies which adopted this approach [26, 27]. Both assessed parent-child agreement for QOL ratings in pediatric cancer populations. These researchers found moderate correlations between child and parent scores and no group differences between their mean scores [26, 27]. It is questionable if test scores displaying moderate correlations can be considered equivalent. Correlation coefficients of at least 0.70 are usually required for a reasonable prediction of individual test scores .
Our goal was to extend knowledge of the factors influencing child-parent agreement in rating child QOL in healthy populations. First, we considered differences in agreement across two different statistical methods. Intra-Class correlation coefficients (ICC) were used to evaluate correlational consistency between child and parent scores on the generic core scales of the UK-English version of the Pediatric Quality of Life Inventory™ 4.0 (PedsQL™) [4, 5], and Wilcoxon median testing to evaluate agreement between child and parent ratings on this measure. Second, we considered the effect of chronological age and domain type (i.e., physical vs. psychosocial aspects) on agreement between children's and parents' reports on the PedsQL™. We assessed parent-child agreement across three age groups (stratified by year in school), and across the Physical Health domain and Psychosocial Health domains. Third, we investigated the relationship between parents' own well-being and their ratings of their child's QOL.
Based on the findings of Eiser and Morse , we predicted that in this healthy sample parent-child correlations would be low to moderate. Furthermore, we expected statistically significant group differences in child and parent median scores, specifically parents' scores would be higher than children's scores due to the over-estimation effects found in previous studies with healthy child populations [13, 18, 19]. In relation to chronological age, we expected that parent-child agreement would increase with child age, based on the findings of previous work [4, 20, 21]. In relation to domain type, we expected that parent-child correlations would be higher for the physical health compared to psychosocial health domains. Finally following on from the findings of Eiser, Eiser and Stride  on the effect of mother's own well-being on their ratings of their children's QOL, we expected that parents' own QOL levels would be correlated to their proxy-reports of child QOL.