The Planning Phase is Crucial
- A DOE effort will fail if not properly planned. The team or individual responsible for the experiment needs to take the time to think through the entire activity.
- Without good planning, the DOE might yield poor results or, even worse, lead to misleading conclusions.
Steps for Designing an Experiment
- Design & Communicate the Objective
- Define the Process
- Select a Response and Measurement System
- Ensure that the Measurement System is Adequate
- Select Factors to be Studied
- Select the Experimental Design
- Set Factor Levels
- Final Design Considerations
Design & Communicate the Objective
- The objective will generally be one of three forms:
- The “Biggest” (maximize the response)
- The “Smallest” (minimize the response)
- The “Closest-to-Target” (hit a target)
Define the Process
- Define the boundaries of the process to be experimented upon.
- This could be just internal processes or it could include the full extended process in which the processes of suppliers and/or customers are studied along with internal processes.
Select a Response and Measurement System
- Responses are the outputs, or the dependent variables, of the process. In analyzing a designed experiment, you can use as many responses as you are willing to measure.
- A good measurement system is one that is accurate, repeatable, reproducible, stable, and linear.
- Taking good samples is a critical aspect of the measurement system. The samples from each experimental run must be representative of the response during that run.
Ensure that the Measurement System is Adequate
- Make sure the measurement system has been calibrated.
- If the measurement system is not repeatable and reproducible, the results of the designed experiment will not be valid.
Select Factors to be Studied
- Factors are the independent variables that will affect the response; select those factors that should have the greatest impact on the response.
- Ensure that it is practical, feasible, and cost effective to select a factor to be studied and to change its level.
Select the Experimental Design
- The type of design is highly dependent on the number of factors to be studied.
- Screening experiments are usually the best design choice early in an experimental sequence when many factors are to be explored.
Set Factor Levels
- Be bold and set the levels at the edges of the operating window for the process for screening experiments.
Final Design Considerations
- Final considerations include:
- Selecting the experimental matrix to use;
- Deciding how to estimate the experimental error; and
- Planning the experiment out so that any external sources of variation are minimized.