OBJECTIVE: The aim of this study was to devise an evidence-based missing data rule for the Quality of Life Inventory-Disability (QI-Disability) questionnaire specifying how many missing items are permissible for domain and total scores to be calculated using simple imputation. We sought a straightforward rule that can be used in both research and clinical monitoring settings. METHOD: A simulation study was conducted involving random selection of missing items from a complete data set of questionnaire responses. This comprised 520 children with intellectual disability from 5 diagnostic groups. We applied a simple imputation scheme, and the simulated distribution of errors induced by imputation was compared with the previously estimated standard error of measurement (SEM) for each domain. RESULTS: Using a stringent criterion, which requires that the 95th percentile of absolute error be less than the SEM, 1 missing item should be permitted for 2 of the 6 QI-Disability subdomain scores to be calculated and 1 missing item per domain for the total score to be calculated. Other, less stringent criteria would allow up to 2 missing items per domain. CONCLUSION: Empirical evidence about the impact of imputing missing questionnaire responses can be gathered using simulation methods applied to a complete data set. We recommend that such evidence be used in devising a rule that specifies how many items can be imputed for a valid score to be calculated.
References
Sterne JA, White IR, Carlin JB, et al. Multiple imputation for missing data in epidemiological and clinical research: potential and pitfalls. BMJ. 2009;338:b2393.
The KIDSCREEN Group Europe. The KIDSCREEN Questionnaires���Quality of Life Questionnaires for Children and Adolescents. Handbook. Pabst Science Publishers; 2006.
Ware JE Jr, Sherbourne CD. The MOS 36-item short-form health survey (SF-36). I. Conceptual framework and item selection. Med Care. 1992;30:473���483.
Varni JW, Seid M, Rode CA. The PedsQL: measurement model for the pediatric quality of life inventory. Med Care. 1999;37:126���139.
Bell ML, Fairclough DL, Fiero MH, et al. Handling missing items in the hospital anxiety and depression scale (HADS): a simulation study. BMC Res Notes. 2016;9:479.
Eekhout I, de Vet HC, Twisk JW, et al. Missing data in a multi-item instrument were best handled by multiple imputation at the item score level. J Clin Epidemiol. 2014;67:335���342.
Peyre H, Lepl��ge A, Coste J. Missing data methods for dealing with missing items in quality of life questionnaires. A comparison by simulation of personal mean score, full information maximum likelihood, multiple imputation, and hot deck techniques applied to the SF-36 in the French 2003 decennial health survey. Qual Life Res. 2011;20:287���300.
Solans M, Pane S, Estrada MD, et al. Health-related quality of life measurement in children and adolescents: a systematic review of generic and disease-specific instruments. Value Health. 2008;11:742���764.
Waters E, Davis E, Ronen GM, et al. Quality of life instruments for children and adolescents with neurodisabilities: how to choose the appropriate instrument. Dev Med Child Neurol. 2009;51:660���669.
Davis E, Reddihough D, Murphy N, et al. Exploring quality of life of children with cerebral palsy and intellectual disability: what are the important domains of life? Child Care Health Dev. 2017;43:854���860.
Epstein A, Leonard H, Davis E, et al. Conceptualizing a quality of life framework for girls with Rett syndrome using qualitative methods. Am J Med Genet A. 2016;170:645���653.
Epstein A, Whitehouse A, Williams K, et al. Parent-observed thematic data on quality of life in children with autism spectrum disorder. Autism. 2019;23:71���80.
Murphy N, Epstein A, Leonard H, et al. Qualitative analysis of parental observations on quality of life in Australian children with Down syndrome. J Dev Behav Pediatr. 2017;38:161���168.
Downs J, Jacoby P, Leonard H, et al. Psychometric properties of the quality of life inventory-disability (QI-Disability) measure. Qual Life Res. 2019;28:783���794.
Epstein A, Williams K, Reddihough D, et al. Content validation of the quality of life inventory-disability. Child Care Health Dev. 2019;45:654���659.
Jacoby P, Epstein A, Kim R, et al. Reliability of the Quality of Life Inventory-Disability (QI-Disability) measure in children with intellectual disability. J Dev Behav Pediatr. 2020;41:534���539.
Terwee CB, Bot SD, de Boer MR, et al. Quality criteria were proposed for measurement properties of health status questionnaires. J Clin Epidemiol. 2007;60:34���42.
Williams K, Jacoby P, Whitehouse A, et al. Functioning, participation, and quality of life in children with intellectual disability: an observational study. Dev Med Child Neurol. 2021;63:89���96.
Fehr S, Leonard H, Ho G, et al. There is variability in the attainment of developmental milestones in the CDKL5 disorder. J Neurodev Disord. 2015;7:2.
