In conceptualizing animal welfare, it is useful to distinguish among three types of concepts. 'Type l' are single, measurable attributes. 'Type 2' are single attributes that cannot be measured directly but can be estimated by correctly combining various contributing attributes. 'Type 3' are concepts involving multiple attributes which are grouped together because they serve some common function, and whose relative importance cannot be established in an entirely objective way. Individuals who treat animal welfare as a type 1 concept may propose single, objective measures of welfare, such as longevity or levels of stress-related hormones; however, this approach rests on judgements, which are not purely objective, about the relative importance of different factors for an animal’s quality of life. Studies of animal preferences and motivation are sometimes seen as an objective way to weight different attributes according to the animals' own priorities, and thus allow animal welfare to be treated as a type 2 concept. However, numerous technical and fundamental difficulties limit our ability to do this. Animal welfare is best seen as a type 3 concept incorporating multiple attributes, with considerable consensus over certain general principles (eg that a high level of welfare implies freedom from suffering) but with value-related disagreement over how these principles should be applied. Because the various attributes cannot be combined in a purely objective way, science is limited in its ability to determine the 'overall 'welfare of an animal and to compare welfare in disparate environments. Instead of attempting to measure' animal welfare, the role of science should be seen as identifying, rectifying and preventing welfare problems.
Fraser, D. (1995). Science, values and animal welfare: exploring the 'inextricable connection'. Animal welfare, 4(2), 103-117.