In Design and analysis of experiment , Random effect is defined as :
An experimenter is frequently interested in a factor that has a large number of possible levels. If the experimenters randomly selects a of these levels from the population of factor levels , then we say that the factor is random .
In random effect model , we are randomly choosing level .
But in response surface methodology class , we have learned that choosing factor level randomly can seriously distort the result of experiment .
My first question is , aren't these two concept of random effect model and response surface methodology contradictory ?
It seems response surface methodology is based on fixed effect model , since experimenter fixed the level . Is it ?
If my study is the first study in that field then how can i fix appropriate levels at the very beginning ? I may have the possibility to choose such levels which do not have optimum result at all . In such case , isn't
trial and errorthe only way to execute the study ?