There are different ways to study a unit and different names applied in different statistical disciplines.
We may be interested in a unit because we intend to generalize from observations on a few units to a statistical assembly of units. Opinion polling[?] and survey sampling provide well known examples of this type of research.
We may be interested in the dynamics of a unit, how its observable characteristics change from time to time. Economic studies of business firms provide an example of this type of research. (See dynamic model[?].)
We may be interested in the internal functioning of a unit which we can characterize as a statistical assembly. This kind of research often involves interference with the unit such as subjecting it to a treatment or even dissection, in some cases. Field experimentation and clinical trials[?] are examples.