Scalability and internal consistency of the German version of the dementia-specific quality of life instrument QUALIDEM in nursing homes - a secondary data analysis

Background: Quality of life (Qol) is a widely selected outcome in intervention studies. The QUALIDEM is a dementia-specific Qol-instrument from The Netherlands. The aim of this study is to evaluate the scalability and internal consistency of the German version of the QUALIDEM. Methods: This secondary data analysis is based on a total sample of 634 residents with dementia from 43 nursing homes. The QUALIDEM consists of nine subscales that were applied to a subsample of 378 people with mild to severe dementia and six consecutive subscales that were applied to a subsample of 256 people with very severe dementia. Scalability, internal consistency and distribution scores were calculated for each predefined subscale using the Mokken scale analysis. Results: In people with mild to severe dementia, seven subscales, care relationship, positive affect, negative affect, restless tense behavior, positive self-image, social relations and feeling at home, were scalable (0.31 <= H <= 0.65) and internally consistent (Rho >= 0.62). The subscales social isolation (H = 0.28) and having something to do (H = 0.18) were not scalable and exhibited insufficient reliability scores (Rho <= 0.53). For people with very severe dementia, five subscales, care relationship, positive affect, restless tense behavior, negative affect and social relations, were scalable (0.33 <= H <= 0.65), but only the first three of these subscales showed acceptable internal consistency (Rho 0.59 - 0.86). The subscale social isolation was not scalable (H = 0.20) and exhibited poor internal consistency (Rho = 0.42). Conclusions: The results show an acceptable scalability and internal consistency for seven QUALIDEM subscales for people with mild to severe dementia and three subscales for people with very severe dementia. The subscales having something to do (mild to severe dementia), negative affect (very severe dementia), social relations (very severe dementia) and social isolation (both versions) produced unsatisfactory results and require revision.