Can we really measure animal quality of life? Methodologies for measuring quality of life in people and other animals

Quality of life (QoL) is an abstract construct that has been formally recognised and widely used in human medicine. In recent years, QoL has received increasing attention in animal and veterinary sciences, and the measurement of QoL has been a focus of research in both the human and animal fields. Lord Kelvin said "When you cannot measure it, when you cannot express it in numbers - you have scarcely in your thoughts, advanced to a stage of science, whatever the matter may be" (Lord Kelvin 1893). So are we able to measure animal QoL? The psychometric measurement principles for abstract constructs such as human intelligence have been well rehearsed and researched. Application of traditional and newer psychometric approaches is becoming more widespread as a result of increasing human and animal welfare expectations which have brought a greater emphasis on the individual. In recent decodes the field of human medicine has developed valid measures of experienced pain and QoL of individuals, including those who are not capable of self-report. More recently, researchers who are interested in the measurement of animal pain and QoL have begun to use similar methodologies. In this paper, we will consider these methodologies and the opportunities and difficulties they present.