Six Sigma Fundamentals: A Complete Introduction to the System, Methods, and Tools

What does the Y= f(x,n) expression mean?

In simple mathematical terms this means that the Y is a function of x and n. In plain language it means that changes in the x (inputs and processes) and some noise (n) will determine what will happen to the Y. (Noise means factors that are uncontrollable or that the experimenter chooses not to control.) In the six sigma methodology, the Y may mean any company aim, such as profits, customer satisfaction, strategic goal and efficiency. The x may mean actions that are important enough in the presence of noise that achieve the strategic goals, influences customer satisfaction, processes variables and so on. The n is a noise that is present in the process, however, it does not effect the x in any significant way. This is called the principle of robustness. Finally, the formula Y = f(x,n) is used primarily in the DFSS approach of the methodology