5.
Process Improvement
5.5. Advanced topics 5.5.9. An EDA approach to experimental design 5.5.9.10. DOE contour plot


What factors go on the two axes? 
For this first item, we choose the two most important
factors in the experiment as the plot axes.
These are determined from the ranked list of important factors as discussed in the previous steps. In particular, the effects plot includes a ranked factor table. For the defective springs data, that ranked list consists of


Possible choices 
In general, the two axes of the contour plot could consist of
In this case, since X_{1} is the top item in the ranked list, with an estimated effect of 23, X_{1} is the most important factor and so will occupy the horizontal axis of the contour plot. The admissible list thus reduces to
To decide between these two pairs, we look to the second item in the ranked list. This is the interaction term X_{1}*X_{3}, with an estimated effect of 10. Since interactions are not allowed as contour plot axes, X_{1}*X_{3} must be set aside. On the other hand, the components of this interaction (X_{1} and X_{3}) are not to be set aside. Since X_{1} has already been identified as one axis in the contour plot, this suggests that the other component (X_{3}) be used as the second axis. We do so. Note that X_{3} itself does not need to be important (in fact, it is noted that X_{3} is ranked fourth in the listed table with a value of 1.5). In summary then, for this example the contour plot axes are:
Vertical Axis: X_{3} 

Four cases for recommended choice of axes 
Other cases can be more complicated. In general, the
recommended rule for selecting the two plot axes is that they
be drawn from the first two items in the ranked list of factors.
The following four cases cover most situations in practice:
