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Performing a Successful Process Capability Study
Tips on Performing a Process Capability Study
A process capability study is performed for two primary reasons:
1. to predict future capability of the process to produce product within specification.
2. to identify process improvement opportunities.
Process capability is a statistical analysis of variation of the process output compared to the allowable specification limits. Cpk is most often used in a manufacturing process to express process capability.
There are common problems that occur in process capability studies that can be overcome with proper planning and implementation before the study begins.
Process Potential Study vs. Process Capability StudyShould process potential (Ppk) be calculated or process capability (Cpk)?
A Ppk calculation is used to define, in statistical terms, what has already happened in a single lot of product. It is often used in the early phases of a project to obtain an estimate of process variation when only a small number of parts have been produced. The data points represent only a “snapshot” of the process and only a small window of its potential; hence the term “process potential study”.
A process capability study (Cpk), on the other hand, results from sampling of a continuous process over planned intervals. It is a prediction of what is likely to occur in the future based on sample subgroups taken over a length of time. It is a method of predicting future capability of the process; hence the term “process capability study”.