This week’s blog concludes our series on process capability and asks which is better: Cpk or Ppk?
Your supplier has sent you the process capability chart you requested. Looks like your supplier is really performing for you. You note that his process has a Ppk = 1.14 and a Cpk = 2.07. Why are those different? Well, it doesn’t matter. The Cpk is above 1.33, which is what you asked the supplier for. You just missed a very important piece of information about your supplier’s performance. Know what it is?
The accompanying video explores Cpk and Ppk in more detail.
The only difference between Cpk and Ppk is the way the process variation is estimated. With Cpk, the standard deviation is estimated from the average range on a range control chart. A range control chart is based on the “within” variation. This is short-term variation. Cpk is sometimes referred to as the short-term capability. With Ppk, the calculated standard deviation is used. This includes all the data at one time in the calculation. Ppk is sometimes called the long-term capability. Does it matter which you use?
- If Cpk is approximately equal to Ppk, the process is in statistical control
- If Cpk is significantly different than Ppk, the process is not in statistical control
So, when you looked at the supplier’s process capability chart and noticed a big difference between Cpk and Ppk, you were given a key piece of information. Your supplier’s process is not in statistical control – and you can’t be sure of what you will get in the future from the supplier.
In addition, if the process is not in statistical control, Cpk and Ppk have no meaning. You cannot be sure of getting similar values in the future because the process is not consistent and predictable.
The reality is that Cpk is a better estimate of the potential of your process. It represents the best your process can do and that is when the within variation is essentially the same as the between variation – the short-term variation estimated from the range chart is the same as the long-term variation represented by the calculated standard deviation. This is what it means to be in statistical control. And if the process is in statistical control, Cpk is essentially the same as Ppk. So, you really don’t need Ppk in this case.
And if your process is not in statistical control, you have something to work on – Cpk and Ppk are pretty well meaningless – except for the fact that a value of Cpk gives you an estimate of what the capability can be if the process is in statistical control.