Example of a contour plot for a response surface design
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In the fertilizer example, you generated a design, supplied the response data, and fit a linear model. Since this linear model suggested that a higher model is needed to adequately model the response surface, you fit the full quadratic model. The full quadratic provides a better fit, with the squared terms for nitrogen and phosphoric acid and the nitrogen by potash interaction being important. The example below is a continuation of this analysis. Now, you want to understand these effects by looking at a contour plot of snap bean yield versus the significant factors - nitrogen and phosphoric acid.

You do not need to re-analyze that response surface model. The worksheet contains the model for the contour plot.

1    Open the worksheet CCD_EX1_MODEL.MTW.

2    Choose Stat > DOE > Response Surface > Contour Plot.

3    In Response, choose BeanYield.

4    Under Variables, in X Axis, choose Nitrogen.

5    Under Variables, in Y Axis, choose PhosAcid.

6    Click OK.

Graph window output

Interpreting the results

The contour plot indicates that the highest yield is obtained when nitrogen levels are low and phosphoric acid levels are high. This area appears at the upper left corner of the plot.

This plot uses a model equation. Ensure that your model is adequate before you interpret the plot.