How to Use the Desirability Function

To use a variable’s desirability function, drag the function handles to represent a response value.

As you drag these handles, the changing response value shows in the area labeled Desirability to the left of the plots. The dotted line is the response for the current factor settings. The overall desirability shows to the left of the row of desirability traces. Alternatively, you can select Optimization and Desirability > Set Desirabilities to enter specific values for the points.

Figure 3.10 shows steps to create desirability settings.

Maximize

The default desirability function setting is maximize (“higher is better”). The High control point is positioned at the maximum Y value and aligned at the high desirability, close to 1. The Low control point is positioned at the minimum Y value and aligned at a low desirability, close to 0.

Figure 3.10 Maximizing Desirability 

Target

You can designate a target value as “best.” In this example, the middle control point is positioned at Y = 70 and aligned with the maximum desirability of 1. Y becomes less desirable as its value approaches either 40 or 100. The High and Low control points at Y = 40 and Y = 100 are positioned at the minimum desirability close to 0.

Figure 3.11 Defining a Target Desirability 

Minimize

The minimize (“smaller is better”) desirability function associates high response values with low desirability and low response values with high desirability. The curve is the maximization curve flipped around a horizontal line at the center of plot.

Figure 3.12 Minimizing Desirability 

Note: Dragging the High or Low control point of a maximize or minimize desirability function across the y-value of the middle point results in the opposite point reflecting. A Minimize becomes a Maximize, and vice versa.