Classification of factors

Factors are elements to make quality. Factors are also called "X" or "Xs".

Error Factor : Disturbance of quality. We can study this factor in the experiment. But we cannot control this factor in manufacturing.
Control Factor : We can control this factor to get good quality.
Signal Factor : In dynamic characteristics, this is the most important X.

Factors and Kinds of Data

Error factor and control factor are used as category data. Signal factor is used as quantity data.

Even if error factor and control factor are quantity data, the information of quantity is not use in the calculation of SN ratio.