Process Capability is the degree to which a system or service meets customer expectations or the  ability of a process to produce defect free products. While you intend to study Process Capability it is recommended that you understand the Normal Distribution curve well. If I add one standard deviation and subtract one standard deviation from the Mean, the area under the curve shall be 68.26%. If I add two standard deviation and subtract two standard deviation it would roughly cover 95.44%. While plus minus three standard deviation will constitute 99.74% of the data.... visit For More Information http://bit.ly/2LN6ily