# Random Effect Model and Response Surface Methodology

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 error the only way to execute the study ?