To answer that, we need to start off with what these capability indices represent. Capability indices characterize a process as compared to a specification. Now the important thing about these capability indices is that they are calculated on the individual measurements, and all of the indices require a process that is in control in order to mean … Continued

Process Control is proactive rather than reactive. The key elements of process control are outlined in this article. What is Quality? Ask people involved with the design and manufacture of a product the following question: “What is Quality?” Many, if not most, of the responses will be some form of the following: “Quality is ensuring that our … Continued

While the construction of control charts is relatively straightforward, often a more difficult question is: how do I know what process characteristics to control in the first place? Clearly, controlling everything is not feasible or a smart use of limited resources. In an earlier Ask the Expert topic, we discussed the philosophy of process control as opposed to inspection … Continued

Let’s take the confusion out of Cpk and Ppk. Just like control charts tell us about the stability of the process, capability analyses tell us about process capability. Capability Indices help us judge whether the process is capable of producing parts that are within specifications. Among the various capability indices, Cpk and Ppk are popular … Continued

Many manufacturing leaders believe that their production personnel use SPC properly, but the evidence suggests otherwise. Statistical Process Control (SPC) is a method that enables manufacturers to prevent problems and to control their production processes. Unfortunately, it is rarely applied correctly, so manufacturers tend to be inefficient, reactive, and even unprofitable. Upon investigating most warranty, … Continued

Looking just at the auto industry, GM has recalled roughly 3 million vehicles this year in the U.S. alone. In 2009, GM recalled roughly 2.2 million vehicles. The burden of warranty costs Recent news offers a reminder that huge opportunities still exist to reduce warranty costs that drain manufacturers’ bottom lines. Consider that during the … Continued

EWMA or Cusum charts are the best charts for detecting process shifts less than 1σ. Typical control charts, such as Xbar & R and X & Mr, indicate whether or not a process is statistically stable. They detect process changes greater than 1.5σ and, because of this, are not the best tool to detect small … Continued

An OC curve allows practitioners to determine a sample size that will result in the detection of those process changes that are of practical significance while minimizing the occurrence of false alarm. When applied properly, SPC provides manufacturers a proven method to increase profitability and achieve a deeper understanding of their processes. Additionally, SPC can … Continued

The purpose of control charting is to regularly monitor a process so that significant process changes may be detected. These process changes may be a shift in the process average (Xbar) or a change in the amount of variation in the process. The variation observed when the process is operating normally is called common cause variation. When … Continued