noise

Factors that influence the performance of a system or product, but are not under control during the intended use of the product. The objective in robust parameter design is to minimize the variability of the product's performance in response to the noise factors. The noise factor levels selected should reflect the range of conditions under which the response should remain robust and must be able to be controlled during the experiment.

An example of a noise factor is the weight of items in a consumer's car, which affects the car's speed (response), but is outside the control of the car manufacturer.