A deterministic model, equation or system is one that assumes a fixed representation of the underlying process, with no element of randomness built in. In other words, it assumes that all the causes of the physical phenomenon are known exactly (without error). Thus, for a given specific set of initial conditions, a deterministic equation will always give the same result, with no margin of error. Most mathematical and physical relations are examples of deterministic models. For instance, the equation relating the radius of a circle to its circumference is deterministic.