Now, let’s consider the usage of Random Constant. If one has experience for Taguchi’s robust design, he will know the noise factors that are uncontrollable factors in system or environments. As you can see, Taguchi’s robust design concept is to determine the design variables to minimize the variance with respect to those noise factors. Let’s consider the following design variable set.
In this problem, the initial meta-models are constructed from the user defined DOE methods. Then, from the meta-model, the approximate variation is evaluated as
where 
and 
are the ith design
variable and variation. Also
is meta-model. In conclusion, Random Constants are Noise Factors
in Robust design and DFSS.