Second， you have to understand thoroughly what Cpk is all about. It‘s a process capability ratiCpk=min{Cpl，Cpu}. It shows how well the process is centered on the target comparing with Cp. So generally people use Cp and Cpk togather trying to figure out the process capability. Furthermore， there are cases where process capability is low but the process is in control，and there are cases where the process is out of control but the process capability is comparative high. These are all related to the variance of the process and how well the process is targeted. There are lots of misleadings in the use of the process capability ratios in the industries.
CP其次，你必须全面理解什么是CPK. 它是一个加工能力比率Cpk=min{Cpl，Cpu}，其显示出该能力是目标中心并优于CP.因此，人们通常将CP和CPK一起使用，并尝试理解其加工能力。此外，这里有很多情形就是加工能力低但加工处于控制之中，而加工处于控制之外时加工能力则相对较高。他们与加工的方方面面联系，同时加工有很强的目的性。在工业CP中使用加工能力的过程中出现了许多误导的情形。
In some industries， such as auto industry， people call the calculation of Cpk as Ppk. As to why people use 32 or more data to calculate Cpk， I did a little research about it. In the industry，people accept Cp 1.33 as a commom sense for existing process which corresponds to 4 sigma variance level. If you use this date to do a little calculatiuon and check the table published by Quality Society of America( I was trying to post that table before， but it didn‘t work. It was all messy. I guess the admin deleted that post)， you will get the number approximately 32. But even 32 is not enough sometimes to get a unbiased estimation of the process capability ratio.
来源:考试网-质量工程师考试
