In order to evade uncontrollable influences such as changes in raw material and tool wear, it is necessary to run experiments in a randomized order.
These variable influences can have a significant effect on the selected variable. If an experiment is not run in a random order, the design of experiment will specify factor effects that are in fact from these variable influences.
Block out Known Sources of Variation: Through blocking, one can screen out the effects of known variables such as shift changes or machine differences.
One can divide the experimental runs into homogenous blocks and then mathematically remove the differences. This increases the sensitivity of the design of experiment. However, it is important to not block out anything one wants to study.
