Health-related quality of life was measured by German age-specific self-report versions and parent-proxy versions of the KINDL-R questionnaire [27 ]. KINDL-R is a generic HRQoL measure that distinguishes six dimensions with reference to the last week: physical (e.g. “I felt ill”) and emotional well-being (e.g. “I had fun and laughed a lot”), self-esteem (e.g. “I was proud of myself”), family (e.g. “I got on well with my parents”), friends (e.g. “I got along well with my friends”), and school (e.g. “Doing the schoolwork was easy”). In the parent-versions analogous items were answered by proxy. Each dimension is measured by 4 items and transformed to a range from 0 (low) to 100 (high). A total score for overall HRQoL from 0 to 100 can also be computed. Parents and adolescents also filled in the 12-item disease-specific obesity module of KINDL-R. To reduce the burden on younger children, for children aged 8-11 years the questionnaire was shortened, and therefore did not include the obesity module.
The KINDL-R was chosen because of its sensitivity to change [8 (link),27 ] and because the disease-specific module as well as German norms are available [28 (link)]. It showed acceptable reliability and validity in different applications [12 ,28 (link),29 (link)]. Cronbach’s alphas in this study were α > 0.80 for the self-reported and parent-reported total scores as well as weight-specific HRQoL scores. Cronbach’s alphas for the generic HRQoL subscales varied from α = 0.54 to 0.80 with the lowest reliability for the friends subscales and values α < 0.70 for self- and proxy-reported self-esteem and school as well as parent-reported emotional well-being and self-reported physical well-being.
HRQoL measures were z-standardized using German norms from a recent representative sample [28 (link)] to allow for easier interpretation of the scores relative to the population and to compensate for age-typical changes in HRQoL. Since population norms for the child self-report version of the KINDL-R are only available from 11 years upwards, for the younger children norms of an 8-12 year-old sample from the KINDL-R manual [27 ] were applied. Because HRQoL was measured every six months over a period of 1.5 years, some children shifted reference category between two measurement points, which may have resulted in discontinuities or leaps in the scores. Moreover, there are no norms available for the disease-specific obesity module. To also allow interpretation of absolute changes in HRQoL, KINDL-R original 0-100 scores were, therefore, analyzed in separate models.
The KINDL-R was chosen because of its sensitivity to change [8 (link),27 ] and because the disease-specific module as well as German norms are available [28 (link)]. It showed acceptable reliability and validity in different applications [12 ,28 (link),29 (link)]. Cronbach’s alphas in this study were α > 0.80 for the self-reported and parent-reported total scores as well as weight-specific HRQoL scores. Cronbach’s alphas for the generic HRQoL subscales varied from α = 0.54 to 0.80 with the lowest reliability for the friends subscales and values α < 0.70 for self- and proxy-reported self-esteem and school as well as parent-reported emotional well-being and self-reported physical well-being.
HRQoL measures were z-standardized using German norms from a recent representative sample [28 (link)] to allow for easier interpretation of the scores relative to the population and to compensate for age-typical changes in HRQoL. Since population norms for the child self-report version of the KINDL-R are only available from 11 years upwards, for the younger children norms of an 8-12 year-old sample from the KINDL-R manual [27 ] were applied. Because HRQoL was measured every six months over a period of 1.5 years, some children shifted reference category between two measurement points, which may have resulted in discontinuities or leaps in the scores. Moreover, there are no norms available for the disease-specific obesity module. To also allow interpretation of absolute changes in HRQoL, KINDL-R original 0-100 scores were, therefore, analyzed in separate models.
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