Firstly what is quality of life, in health usually expressed as QoL. A search of the internet (Dr Google!) came up with a number of definitions. One of the more detailed is the paper by Lesley Fallowfield, Professor of Psycho-oncology, Brighton & Sussex Medical School, University of Sussex
How do you measure QoL in people with dementia? It would seem with difficulty and not reliably according to a recent paper by Bowling et al (2015), who systematically conducted a narrative review of dementia-specific measurement scales and concluded there was a lack of consensus on measuring the QoL in dementia.
Bowling A, Rowe G, Manthorpe J, et al. Quality of life in dementia: a systematically conducted narrative review of dementia-specific measurement scales. Aging & Mental Health [serial online]. 2015;19(1):13-31.