Use a contour plot to help you visualize the response surface. Contour plots are useful for establishing desirable response values and operating conditions.

A contour plot shows how a response variable relates to two continuous variables based on a model equation. That is, the contour plot represents, in two dimensions, the functional relationship between the response and the variables. Points that have the same response are connected to produce contour lines of constant responses.

Because a contour plot shows only two variables at a time, any extra variables are held at a constant level. Thus, the contour plots are for fixed levels of the extra variables. If you change the hold levels, the response surface changes as well, sometimes drastically.

Contour plot does not use the data in the worksheet. Instead, Minitab estimates the contours based on a stored model. You must fit a model with two or more covariates before you can generate a contour plot. Contour plots are accurate only if the model represents the true relationships.

For the cleaning efficiency data, previous analysis showed that pressure and time have quadratic effects on Efficiency, while concentration only has a linear effect. Therefore, it makes sense to hold concentration fixed at its low and high levels and compare the plots. The interpretation of the contour plots is as follows:

·    Pressure versus Time (Conc = 2.0): This plot shows how pressure and time are related to cleaning efficiency when concentration is at its low level of 2.0. The darkest green area indicates the contour where the response is the highest (99). To maximize cleaning efficiency, you would choose settings for pressure and time in the center of the right side of the plot. For example, you could choose a pressure around 200 and a time around 4.5.

·    Pressure versus Time (Conc = 7.0): This plot shows how injection pressure and time are related to cleaning efficiency when concentration is at its high level of 7.0. The darkest green area indicates the contour where the response is the highest (105). To maximize cleaning efficiency, you would choose settings for pressure and time in the center of the right side of the plot. For example, you could choose a pressure around 200 and a time around 4.5

·    Low Concentration versus High Concentration: The plots indicate that as concentration increases from its low to high level (2 to 7), Efficiency also increases. Maximum Efficiency of approximately 105 occurs around Pressure = 200, Time = 4.5, and Conc = 7.

The contours correspond to a minimax response surface.