Measurement of this ‘elusive concept’ is problematical. (When concepts are elusive measurement is generally difficult!) To circumvent time consuming (and often fruitless) debate over the question what ‘quality of life’ really is, our approach to this issue is pragmatic. Each AQoL (or other) instrument proposes a set of attributes which is postulated to be of importance for the individual or for policy makers. To the extent to which the descriptive system captures what individuals and policy makers wish to measure, the instrument is successful. To the extent to which it omits important elements, or describes them inadequately, the instrument is defective in a particular context. This pragmatic approach underlies the need for users to assess whether or not a particular instrument satisfactorily meets their needs. There is no perfect instrument.
A particular problem is that QoL may be visualised and articulated a number of ways. A person with a particular musculoskeletal problem may find difficulty in carrying out particular tasks and this may or may not have an effect upon their life. (The effect of arthritis in the hands of a piano removalist and piano player is quite different.) The WHO defined three different perspectives.
‘Impairment’ is the medical condition afflicting a person and may be described in medical terms (for example, arthritis).
‘Disability’ occurs when this affects the person’s ability to function in a particular way, for example the person cannot easily move their fingers, bend or stretch.
‘Handicap’ refers to the impact upon a person’s life in a social context, for example, a person may be unable to leave their house, play the piano or carry out household tasks.
Because we postulated that ‘utility’ will be most closely related to behaviour in a social context the AQoL suite of instruments adopts the perspective of handicap. However none of the classifications is complete and the AQoL instruments are supplemented, where necessary, with concepts from the other perspectives. For example, pain has an intrinsic disutility in addition to any effect upon a person’s functioning.