Is defining entity/object type populations arbitrary?

I have been saying (see my video lecture on Subtypes/Supertypes on YouTube) that defining populations is essentially "arbitrary", a choice made by the designer in modeling some user domain.  This is to contrast with a common view that there is one correct model and it is the good designer who can find it!

Fabian Pascal commented:  I would not refer to it as "arbitrary", but pragmatic -- to incorporate perceptions and serve application needs of users.

I dislike the word "arbitrary" too. Fabian is absolutely correct. Sometimes it may seem arbitrary to others, but it is actually a choice made by the designer(s) based on the purpose(s) of the model. Since the world presents itself to us as only made up of instances (individual things), the modeler must choose how to define the grouping of things into populations or classes. This is part of the abstraction process in modeling. As an example, one may be interested in animals and a cow would be an instance. But a dairy farmer probably wants to keep information on each individual cow. As another example, a designer may define a population of Employee, but that may include or exclude applicants, furloughed, part-time, contract, retired, etc. depending on your purpose.

.. To arrive at an efficient representation of the user domain, we group individual instances into populations so that we can define and manipulate its members in the same way. Efficiency goes for (1) our own human ability to grasp an understanding of our world, (2) our ability to define relationships, attributes, constraints, etc. which apply uniformly to members of a population, and (3) our processing the representation of that world in our databases.

.. Thanks, Fabian, for calling me out on that one. I am not sure I would use the term "pragmatic" since that implies the designer's choice is matter-of-fact, obvious, cut and dried, unequivocal, or settled. However, there may still be disagreement and alternative designs. Anyone suggest a better word?