4. An approach to selecting factors and their levels and ranges
The NCM and Pareto Chart analysis facilitated the process of identifying the key areas of the manufacturing process that required particular attention, also helping in a better definition of the problem. These analyses, together with brainstorming with key experts, helped in listing down the controllable, uncontrollable and noise factors that affect the problem. At a second stage, and after all the factors have been identified, brainstorming further reduced the number of factors, simplifying the subsequent DoE analysis Table 2 shows the final list of identified controllable factors, which corresponds to the first part of step II of DoE in Table 1
The second part of step II of DoE is to select factor levels and ranges, which are inputs to the experiments that determine the magnitude and direction of each factor's effect on the response variable(s), therefore directly affecting the results. If the factor levels and ranges are not correctly chosen, the subsequent statistical analysis and final recommendations might be misleading
Selection of factor levels and ranges has to go through an iterative process. The first iterations of factor levels and ranges may contain experimental design with too many experimental runs and might not be practically feasible. Therefore, it may require reducing the experimental runs based on available resources and experimental objectives. The tools used for a comprehensive selection off actor levels and ranges are experimental objectives, theoretical knowledge, expert opinion, process knowledge, available resources previous experimental results and performing pre-experimental runs (Czitrom, 2003; Montgomery, 2008)