Review of manuscript:  KINDL Quality of Life Questionnaire in Serbia. Referent Values for Healthy Children and Adolescents by Dr. Dejan Stevanovic.

The aim of this study is to present the reference values of the Serbian version of the KINDL using a representative population based sample including children, parents and parents.

The manuscript is of interest in the field of Quality of Life and for future applications of the Serbian KINDL. Nevertheless, to be published the manuscript need to be improved regarding English writing and clarification of methods, results and finding-conclusions.

- Abstract

The abstract should be revised to be more comprehensive and show  the relevance of this study in the field of health related quality of life measurement and application in evaluative studies.

English should be revised.

“The objective of this report was to present the referent values...” should be changed for “The objective of this study was to present the reference values...”

- Title and context-introduction

The author mentions the term Quality of Life and in the introduction mentions that the KINDL is a quality of life (QoL) questionnaire, considered as a psychological construct including physical, psychological and functional aspects of well-being and daily function.

The author should consider that Health Related Quality of Life is different from a broader sense of Quality of Life and argument why it includes a psychological construct instead of a  multidimensional health construct.

Briefly a more detailed description of potential uses of this study should be made in the introduction, explaining why norm values are used in the measurement of mental health and HRQL instruments and also potential uses in future applications of the KINDL normative values in Serbia .

The aim of the study is not clear enough. It would be interesting to add comparison with original German version of the KINDL or other normative values of the KINDL in other countries.

- Methods

More clarification of the sentence “The Serbian KINDL possesses satisfactory measurement characteristics as a screening QOL questionnaire” and include additional information of uses of the instrument in the field of mental health or health services evaluation.

The author should reference in the text either in methods or discussion the characteristics of a tool to be able to be used for group comparison and as individual screening instrument  in terms of psychometric properties (Cronbach alpha, Intraclass Correlation, Validity and Sensitivity to Changes).

Even if published in other papers, more description of the sample should be made specially when mentioning representativeness and random sampling. Is the sample representative of children in Serbia, of adolescents in Serbia? And parents? In discussion section, any limitation of the sampling method and final sample obtained should be described.

Regarding the approach of analysis, internal consistency is an additional aim not mentioned in the introduction. Children and adolescents scores are presented aggregated and not separately for boys and girls. Reference values are given for each age (from 8-9, 9-10, 10-11, 11-12, 12-13, 14-15, 16-17) for the KINDL self-reported and parent’s version. This introduces small numbers when stratifications are made by gender and introduces limitation regarfind representativeness of these subgroups and specially when mean comparison of means for each domain and global score are made by gender and age, this may have introduced limitation due to lack of statistical power.

It would be recommended to groups children and adolescents (age groups) scores differently by gender  to present comparison of mean scores and 95% confidence intervals and also percentiles, similar to the original version of the KINDL, other published studies of HRQL measures such as the KINDSCREEN. 

It would be interesting to show the scores of normative/ reference values of the original KINDL questionnaire to see differences in scores in Serbia and Germany, perhaps in a figure.

- Results

More detail should be included regarding results of the study shown in table. Also regarding sample characteristics.

- Discussion and conclusion

The author should include more references to other studies in the field and discuss the potential uses of the results of this study in more detail.

Regarding interpretation strategies of HRQL measures, more detail should be given of the strategy of reference values for potential users of the instrument in research and clinical practice.

Reasons for not finding differences in HRQL using the Serbian version of the KINDL should me mentioned including references of studies showing gender differences in children’s and adolescent’s physical, psychological, social or well-being.

During some years my field of research was the development of questionnaires of health related quality of life for children and adolescents including the psychometric properties and interpretation strategies of these intruments for potential users. I participated in the adaptation and validation of the Spanish version of the KINDL and published in the field of HRQL.

My actual field og interest involves the measurement of HRQL instruments for the assessment of effectiveness of health services in adult population.