Design of experiments (DOE) is a powerful tool that can be used in a variety of experimental situations. DOE allows for multiple input factors to be manipulated determining their effect on a desired output (response). By manipulating multiple inputs at the same time, DOE can identify important interactions that may be missed when experimenting with one factor at a time. All possible combinations can be investigated (full factorial) or only a portion of the possible combinations (fractional factorial). Fractional factorials will not be discussed here.
When to Use DOE
Use DOE when more than one input factor is suspected of influencing an output. For example, it may be desirable to understand the effect of temperature and pressure on the strength of a glue bond.
DOE can also be used to confirm suspected input/output relationships and to develop a predictive equation suitable for performing what-if analysis.
Acquire a full understanding of the inputs and outputs being investigated. A process flow diagram or process map can be helpful. Utilize subject matter experts as necessary.
Determine the appropriate measure for the output. A variable measure is preferable. Attribute measures (pass/fail) should be avoided. Ensure the measurement system is stable and repeatable.
Create a design matrix for the factors being investigated. The design matrix will show all possible combinations of high and low levels for each input factor. These high and low levels can be generically coded as +1 and -1. For example, a 2 factor experiment will require 4 experimental runs:
|Input A Level||Input B Level|
|-1 Level||+1 Level|
|Temperature||100 degrees||200 degrees|
|Pressure||50 psi||100 psi|
|Experiment #1||100 degrees||50 psi||21 lbs|
|Experiment #2||100 degrees||100 psi||42 lbs|
|Experiment #3||200 degrees||50 psi||51 lbs|
|Experiment #4||200 degrees||100 psi||57 lbs|
|Input A Level||Input B Level||Interaction|
Conduct and analyze up to three factors and their interactions by downloading the 3-factor DOE template (Excel, 104 KB).
More complex studies can be performed with DOE. The above 2-factor example is used for illustrative purposes. A thorough discussion of DOE can be found in Juran’s Quality Handbook.
Contributed by Dean Christolear.