The Psychology of Quality and More
Design of Experiments (DOE): How to do it
How to do it
When there are many factors, reduce the list of those that are to be varied by selecting those known to affect the result and those whose effects are uncertain. It might also be appropriate to perform a series of experiments, starting with a smaller subset of 'high-probability' factors.
There will be fewer trials to perform and the subsequent analysis will be easier if very few levels of each factor are selected. Two levels are sufficient for many cases, as this will show whether changing the factor level changes the experimental result. Three or more levels can be used to check for a non-linear response.
Select the levels to be representative of the range of normal values. Thus they should be sufficiently separated to be able to identify changes over the normal operating range, but not so spread as to meet any boundary effects, such as where a liquid boils.
Ensure the factors can be controlled and measured at these levels. If they cannot be controlled, then it may be sufficient to measure them and sort them into ranges.
Fig. 1. Building an experiment
The decision on how many trials to perform may include economic factors, such as time, effort and cost. For example, crash-testing of vehicles is expensive and time consuming and is impractical to do too often.
When trials are selected, check that they are balanced, with the different levels of each factor occurring the same number of times. Also check for orthogonality, with each pair of factors having each combination of levels occurring the same number of times (as illustrated above).
Consecutive trials should not have any chance of affecting one another; if this could happen, perform trials in random order.
Results may be recorded in a simple table, such as illustrated, which shows one trial per row, with levels and results on the same row. This will help analysis, as results may be visually correlated with the selected factor levels.
Where there are more than two levels, this will result in lines through more than two points. If the lines are not straight, then this indicates a complex effect.
And the big