The representation of physical effects, in a dynamic physical system, in terms of admittedly over-simplified equations containing arbitrary constants, rather than representing them by equations in which the constants have physical meaning and are theoretical consequences of the system's dynamics. Use of such techniques is dictated by lack of adequate knowledge of the physics of the system and often is a matter of mathematical convenience. The validity of the representation must be judged by the faithful-ness of the representation to measurements on the system.