Process Capability Introduction

Introduction

Process capability is an important concept for industrial managers to understand. The challenge in today’s competitive markets is to be on the leading edge of producing high quality products at minimum costs. This cannot be done without a systematic approach and this approach is contained within what has been called “statistical quality control” or “industrial statistics.” The segment of statistical quality control (SQC) discussed here is the process capability study.

So why is process capability so important? Because it allows one to quantify how well a process can produce acceptable product. As a result, a manager or engineer can prioritize needed process improvements and identify those processes that do not need immediate process improvements. Process capability studies indicate if a process is capable of producing virtually all conforming product. If the process is capable, then statistical process controls can be used to monitor the process and conventional acceptance efforts can be reduced or eliminated entirely. This not only yields great cost savings in eliminating nonvalue added inspections but also eliminates scrap, rework and increases customer satisfaction. The benefits of performing process capability studies are certainly worth the effort in the long run.

After a process capability study has been performed, a process will be classified as either capable or incapable. When the process is not capable of producing virtually all conforming product, the process is said to be incapable and acceptance sampling procedures (or 100% inspection) must remain part of the process. Figure 1 shows the process output when compared to the specification limits for a typical incapable process.

Processes may also start out as capable but changes over time to have more variability. In addition, the process mean may also shift placing the process too close to one of the specification limits. Both increases in process variability and shifting of the mean may result in once capable processes becoming incapable.

Figure 2 illustrates a once capable process that, over time, has become incapable due to increased variation. Figure 3 illustrates the effects of process  capability due to shifting in the mean with variation remaining the same.