A model is an idealized representation — an abstract and simplified description — of a real world situation that is to be studied and/or analyzed. Models can be classified in many ways. A mental model is an individual's conceptual, unstated, view of the situation under review; a verbal or written model is a description (a model) of one's mental model; an iconic model looks like what it is supposed to represent (e.g., an architectural model of a building); an analogue model relates the properties of the entity being studied with other properties that are both descriptive and meaningful (e.g., the concept of time as described by the hands and markings of a clock); a symbolic or mathematical model represents a symbolic representation of the process under investigation (e.g., Einstein's equation e = mc²). Decision problem; Descriptive model; Deterministic model; Mathematical model; Normative model; Predictive model; Prescriptive model; Stochastic model.