CpK Defined

There are many different types of ways to help measure whether a product is successful or not. However, one of the most widely used methods is process capability. The general meaning of process capability is the ability of a process to meet its intended purpose designated by the process definition or management. In layman’s terms, process capability basically helps determine if a process is working sufficiently to create a useful and successful product.  Many times the process capability is measured using the process capability index also known as cpk. The results are can often be depicted within a histogram type chart so they are more easily interpreted and read. When truly determining the process capability there are two parts that are taken into consideration: the variability of the output and that variability compared with the projected specifications for that output.

An example in which Process Capability is Helpful

An industrial company that manufactures brake pads for personal auto use is receiving some complaints from franchise corporations that the shipping time frame is taking way to long. The customer service team looks into this issue and does notice a trend of increased shipping time on all brake pad orders. The brake pad company decides to conduct a study using the process capability model. In order to do this, the entire process of how the brake pads leave the facility to how they arrive at their destination is studied so as to find where the lagging time is being spent. First, the study begins with the process of getting the brake pads to the boxes, then moves to how the pads are loaded upon the transport trucks and then to the delivery drivers and the routes taken for delivery. By analyzing all the necessary process steps and considering the time and tactics used with each step it is easier to identify where the ship time is lagging. In essence the process capability index (cpk) will measure how close the process is to its target ship time and how consist it is to help set an average performance level while also indicating where the lags in time are taking place.

Important facts regarding Process Capability

In order to justify valid capabilities, all data included within the study must be from an “in-control” process while also taking both standard deviation and the mean into consideration. Your capability statistics will not be useful or valid if the processes are not controlled. There are two types of errors that can result with the interpretation of control charts. The first error is a type I error and it occurs when a specific process is said to be “out of control” when actually the process is “in control.” The other error is titled a type II error and that occurs when a process is believed to be “in control” when actually the process is “out of control.” You will notice these errors are the exact opposite of each other, however, each error notes some sort of wrong assumption that must be rectified. Lastly, it is often common practice to utilize both histogram charts with control charts together to paint a more accurate picture of what the data is resulting in.