Consider the following guidelines when you define the design.
A signal factor is an input to the system that is intended to change the response. Signal factors are used in dynamic experiments, in which the response is measured at each level of the signal. The goal of the experiment is to improve the relationship between the signal factor and the response.
An example of a signal factor is gas pedal position. The response, the car's speed, should have a consistent relationship with the amount of pressure applied to gas pedal.