Factors are the inputs to the process. Factors in the experiment are controlled and set to levels prescribed by the design. It is up the experimenter to choose which factors are varied over what ranges.
Once the goals for the responses are known work through the following steps.
Make a list of all the factors that could influence the responses.
Group the factors into categories such as: most likely, somewhat likely and least likely to influence the responses.
Note if the factor is easily controllable or difficult to control.
Decide which factors to include in the experiment.
The remaining factors can be held constant or allowed to vary. If allowed to vary, decide if their actual setting can be monitored and recorded.
Once the factors have been picked, the ranges they cover must be chosen. The ranges should be large enough to cause a change in the response(s), but not so large that the process will “fall off a cliff” and produce unusable data.
After this it is time to determine how many runs it will take to reach he goals of the experiment.