Extending Capability Animation with an add-in: Interactively exploring the impact of proposed process changes
Mar 20, 2014 1:08 PM
In quality improvement, it’s common to talk about the "voice of the process" (intrinsic variation in the outputs of an in-control process) and the "voice of the customer" (specification limits that express the range of output values customers will not be unhappy with). A capability analysis compares these two "voices," and summarizes the result via the well-known capability indices Cp and Cpk. These indices are both defined so that larger values correspond to better overall performance. Cp compares the process variation to the width of the spec window, and Cpk is a measure of how well this variation is centered on the target value (mid-way between the spec limits).
When calculating process capability from a normal fit to your data, the Capability Animation feature (available in JMP via Analysis > Quality > Capability or Analysis > Distribution > Capability Analysis) allows you to assess the relationship between Cpk and a proposed shift in the mean or a change in the specification limits.
For example, in Figure 1 we see a normal fit to output from a process that has a standard deviation of 3.51, and for which there is a lower spec limit (LSL) of 70 and an upper spec limit (USL) of 100. Figure 1 also shows the effect of shifting the original mean (72.8, in gray) to a new, higher value (78.12, in blue) that is closer to the target. So if we were (somehow . . . ) able improve the centering of the process in this way, we could expect the Cpk value to increase from 0.27 to 0.77. Note that in this case, the process variation is assumed to be unchanged, so the value of Cp is unchanged also.
My colleague William Zhou and I developed an add-in that extends this built-in Capability Animation to include the option of exploring how changes in the process variation may affect both Cpk and Cp. This add-in is now available in the JMP File Exchange.
You can install the add-in by double clicking on it, and then use it by selecting Add-Ins > Capability Animation. Figure 2 illustrates that reducing the standard deviation of the process from 3.51 to 1.96 (without shifting the mean) increases the Cpk from 0.27 to 0.48 and Cp from 1.42 to 2.54.
This new add-in allows you to perform what-if analysis to determine how any changes to the process mean, process standard deviation or specification limits are likely to impact the values of Cpk and Cp. This can help you to focus your process improvement efforts in the best way, either by re-engineering the process itself, or by renegotiating requirements with customers to get new spec limits.