When input variables are held constant and variation occurs in the response variable, the condition...

The correct answer is C. an experimental error . In the DOE terminology used in CSSBB materials, experimental error is defined as the variation in the response variable beyond what has been accounted for by the factors, blocks, or other assignable sources in the experiment . This means that when the input variables are held constant but the response still changes, the unexplained variation is treated as experimental error.

The CSSBB instructor material also discusses residual error as the difference between observed and predicted values and notes that it can be variation in outcomes under virtually identical test conditions . That idea is consistent with the question: the factors are fixed, but some variation still remains because of natural noise, measurement variation, or other uncontrolled influences. A Type I error is a hypothesis-testing concept, not a DOE condition. A discrete variable is simply a type of data. Randomization is a design principle used to reduce bias, not the name for unexplained response variation. Therefore, under CSSBB DOE terminology, the correct answer is experimental error .