A

**statistical model**is used in applied statistics. Three basic notions are sufficient to describe all statistical models.

- We choose a statistical unit which we will observe directly. Multiple observations of the same unit over time is called longitudinal research. Observations of a variety of statistical attributes is a common way of studying relationships among the attributes of a single unit.
- We may really be interested in a statistical population (or set) of similar units rather than in any statistical unit per se. Survey sampling offers an example of this type of application.
- Our interest may be focused on a statistical assembly where we examine functional subunits of the statistical unit. Physiology is an example in which we probe the organs which compose the unit. A common model for this type of research is the stimulus-response model.