Welcome to Capability Studies
Process Capability is required by TS 16949 and is part of the typical APQP process. Process capability compares the output of an in-control process to the specification limits by using capability indices. The comparison is made by forming the ratio of the spread between the process specifications (the specification "width") to the spread of the process values, as measured by 6 process standard deviation units (the process "width"). Formulas used to calculate capability are:
Cpu = (USL - mean) / 3 * standard deviation
Cpl = (mean - LSL) / 3 * standard deviation
Cpk = min {Cpu, Cpl}
The Cpk is the most commonly used index for calculating capability, however some have found that the Ppk index is actually better. The Cpk is used to gauge the potential capability of a system, or in other words, a system’s aptitude to perform. The Ppk (and relative pp and Pr) actually measure the performance of the system. To determine which of the indexes to use, determine whether you want to analyze the actual performance (Ppk) or the potential capability of the system (Cpk). Cpk is calculated with sigma equal to 3, which is an estimated sigma. Calculating Ppk uses a calculated sigma from the individual data.
Understanding the need for capability indexes is an important part of the analysis. The indexes help to determine the system’s ability to meet specifications. The problem found in using the Cpk, is that it does not account for the average. This will not only let you see what the systems potential is, not necessarily what the average of the system looks like. For example, a Cpk of 1 indicates that the system is at least 99.73% within its specifications. |