An engineer's job is to design a product/process to perform its function close to ideal at all customer's conditions. As he can choose any system and set parameter's levels of the system,those variables are called control factors. All customer's conditions are either signals or noises. The functionality means to minimize the effects of noises by keeping the effects of signal/signals at desired.
The traditional way of designing robustness is to use response function to meet the target function as far as possible. The way may be said the first order moment methods. Instead, quality engineering Taguchi way is to use signal-to-noise ratio as the measure of functionality. The way may be called the second order moment methods. The difference between the two methodologies is discussed in this article.