210 CHAPTER 8: Design of Experiments: A Key to Innovation in Nanotechnology Local control or blocking is a technique that is used to segregate an uncontrolled but known variation in an experiment not associated with the treatment effect. The blocking should be designed to have maximum variation among blocks (heterogeneous between blocks) but to have minimum variation with blocks (homogeneous within blocks). Replication refers to the replication of a treatment combination. It is needed for a specific degree of precision for measuring treatment effects. The reader should be aware that replications are not multiple readings. Replication requirements are stringent: to assure a proper replication, experimenters must reset every condition in the experiment. If the treatment combinations are not reset, the errors in the multiple readings are not independent. This, in turn, leads to the violation of the randomization principle. 8.2 OFAT: THE PREDOMINANT METHOD USED IN