5.6.2.9. Next Step: DOE Contour Plot

The DOE contour plot is used to determine the best factor settings for the two most important factors in the next iteration of the experiment.

From the previous plots, we identified X2 (solute) and X7 (horn depth) as the two most important factors.

We can make the following conclusions from the DOE contour plot.

The best (high light intensity) setting for X2 is "-" and the best setting for X7 is "-". This combination yields an average response of approximately 224. The next highest average response from any other combination of these factors is only 76.
The non-linear nature of the contour lines implies that the X2*X7 interaction is important.
On the left side of the plot from top to bottom, the contour lines start at 0, increment by 50 and stop at 400. On the bottom of the plot from right to left, the contour lines start at 0, increment by 50 and stop at 400.
To achieve a light intensity of, say 400, this suggests an extrapolated best setting of (X2, X7) = (-2,-2).
Such extrapolation only makes sense if X2 and X7 are continuous factors. Such is not the case here. In this example, X2 is solute (-1 = sugar and +1 = glycerol) and X7 is flask clamping (-1 is unclamped and +1 is clamped). Both factors are discrete, and so extrapolated settings are not possible.