Process Performance Graph
The Process Performance Graph is a four-quadrant plot of capability versus stability. Each process for which specification limits are provided is represented by a marker. The marker type depends on if the process violates guidelines for the Target Index and Cp values. A left-pointing triangle indicates the process is below target, a circle indicates the process is on target, and a right-pointing triangle indicates the process is above target. An open marker indicates that the variation of the process is adequate and a filled marker indicates that the variation of the process could be reduced. A complete description of the marker combinations is provided in Table 22.1. Additionally, if you define importance values for the processes, the markers on the Process Performance Graph are sized by importance.
|
Graph Marker |
Target Index |
Cp |
|---|---|---|
|
On Target |
Adequate Variation |
|
On Target |
Reduce Variation |
|
Above Target |
Adequate Variation |
|
Above Target |
Reduce Variation |
|
Below Target |
Adequate Variation |
|
Below Target |
Reduce Variation |
On the graph, the horizontal coordinate represent the stability index of the process and the vertical coordinates represent the capability of the process, given as Ppk. The graph is divided into four quadrants based on the following default boundaries:
– A stability index that exceeds 1.25 indicates that the process is unstable.
– A Ppk that is smaller than 1.33 indicates that the process is not capable.
Additionally, there is a red line on the graph at 1.33 divided by Stability Index that indicates where the Cpk value is 1.33. This line categorizes the processes in the unstable and incapable quadrants in terms of how they might be fixed. Processes below the red line are fixable by special causes; processes above the red line are fixable by reducing variability. Selecting points in the graph selects the corresponding processes in the Summary table.
Tip: Place your cursor over a point in the Process Performance Graph to view the Quick Graph for that process. Click the Quick Graph to add it to the report window.