Cpk vs Ppk - Part 5

Here the sample data collected from the SPC activities, subgroup size = 15. Tape_1 refer to 0.53” tape and Tape_ 2 for 0.46” tape.

From the raw data, without doing any statistical analysis, believe you can tell the adhesiveness of Tape_1 is stronger than Tape_2. Common sense lah… Tape_1 is wider than Tape_2.

Now, look at the chart below, without doing any complicated calculation, you can tell that Tape_1 will have higher Cpk value than Tape_2. Common sense lah… Mean value of Tape_1 is higher than Tape_2.


Ok, now the capability analysis of Tape_1 and Tape_2.




From the data spread, both data set having more or less equal standard deviation, Cpk / Ppk Tape_1 = 2.50/2.47 and Cpk/Ppk Tape_2 = 1.48/1.51. Oh no… Cpk and Ppk Tape_2 is not meeting the industrial standard. How can we conclude that the process of putting smaller tape on the PCA is not capable therefore we should put in more effort to increase the Cpk and Ppk value at least to the industrial standard ?

There are two possible way to increase the Cpk / Ppk value if you want it. You can either increase the adhesive strength or reduce the standard deviation of the smaller tape. Anyway, Tape_1 and Tape_2 having similar material property. Practically it is not possible to make the smaller tape stronger than the larger tape. Now, the only solution to increase the Cpk / Ppk is to reduce standard deviation. The chart above shows that standard deviation of Tape_1 and Tape_2 are more or less equal, it gives me an impression there is no abnormally or special cause happen. Yes, you can try to minimize the process variation, but I m not sure how much we can achieve and how much impact to the Cpk or Ppk value.

Ok, after the long story, is time for conclusion. Cpk and Ppk is a measure for process capability, 1.67 is an industrial standard benchmark. I don’t have any objection on this. When we decided to choose the index as a guide line to measure the process capability, we have to understand the background and nature characteristic of the process. Of cause, we hope everything can be 1.67, but unfortunately not everything can be done. However, it is not the end of the world if 1.67 cannot be done. As a manager, we must have the knowledge to weigh the situation and set practical target.

My personal opinion, Cpk/Ppk 1.48/1.51 is good enough for Tape_2 because I know the natural characteristic of the process. I don’t worry too much about the process shift because overall distribution is far away from the LSL. What I suggest is to continue monitor the process stability through SPC.

Last update : 19/06/08