The construct of quality of life (QOL; e.g., Cummins, 1997, Felce, 1997, Felce and Perry, 1995, Felce and Perry, 1996, Schalock, 1990 and Schalock, 2000) is increasingly being used as framework for planning interventions and evaluating outcomes for individuals with intellectual disability (ID), and for improving the services and supports provided to persons with ID (Gómez et al., 2013, Luckasson and Schalock, 2013, Schalock et al., 2008 and van Loon et al., 2013). These used require both the development of empirically validated models of QOL and of valid scales that allow its measurement in individuals with ID (e.g., Cummins, 2005) and the use of assessment strategies that reliably and validly assess the individual's life conditions and circumstances.
One of the most validated quality of life models (Schalock & Verdugo, 2002) includes eight core domains structured in three main factors: (1) Independence that is composed of personal development and self-determination; (2) Social participation composed of interpersonal relations, social inclusion, and rights; and (3) Well-being composed of emotional, physical, and material well-being. This model has been empirically validated across different cultures and countries (Jenaro et al., 2005, Schalock et al., 2005 and Wang et al., 2010).
For a valid measurement of one's quality of life, culturally sensitive indicators for each domain are used (Schalock and Verdugo, 2002 and Schalock et al., 2005). This approach to the conceptualization and measurement of quality of life moves the field beyond the previous emphasis on subjective indicators and objective indicators to a standard set of domain-referenced indicators that are used as the basis for the assessment. In addition, however, there is also a need to develop assessment strategies that move beyond the use of proxies and one that focuses on the use of the same indicators that are assessed on the basis of either self-report or the report of others (e.g., Cummins et al., 2010, Lucas-Carrasco and Salvador-Carulla, 2012, Petry et al., 2009a and Petry et al., 2010). Using the same indicators for both the self-report and report of others versions of a QOL scale, one can evaluate more validly the potential differences between one's self-report and that provided by others such as family members or direct support staff.
To date, there has been conflicting information regarding the concordance between a self-report and the information provided by others (e.g., Schalock and Keith, 1993, Schmidt et al., 2010 and Zimmermann and Endermann, 2008). Some have found a very low or even the absence of any agreement (Rapley et al., 1997 and Zimmermann and Endermann, 2008). Conversely, other studies (e.g., McVilly et al., 2000, Schalock and Keith, 1993, Schmidt et al., 2010 and Stancliffe, 1999) provide evidence for moderate to strong associations between a self-report and the information provided by others. Many of these differences might well be related to the assessment strategy used as opposed to actual differences (e.g., Petry, Maes, & Vlaskamp, 2009b).
The primary purpose of the present study was to investigate the inter-rater reliability of report of others and the concordance between self-report and report of others related to the assessment of QOL of adults with ID. Quality of life was measured with the Personal Outcomes Scale (POS; van Loon, Van Hove, Schalock, & Claes, 2008). Psychometric properties of the POS are described in the Methods Section and additional information can be found in Claes, Van Hove, van Loon, Vandevelde, and Schalock (2009) and van Loon et al. (2008). POS allows for the evaluation of the eight core domains and the three main factors of the Schalock and Verdugo's model of QOL. These domains and main factors are assessed via two different POS versions: self-report and report of others. Self-report is administered to the person with ID and results in the assessment of client's point of view on his/her QOL. Report of others may be compiled with two different assessment strategies: (1) administration to a caregiver of the individual with ID to obtain a third-party-point-of-view regarding the client's QOL; and (2) administration to a second caregiver who are asked to respond on the individual's with ID behalf to obtain an estimation of the client's point of view. This information is useful if client is not be able to answer to the self-report.
