Rick Pastor

I have made the same calculation for all three sets of data:  The results are below:  In JMP, the distributions (all 50 points) for sets 1, 2, and 3 appear different. 
 
Now, I have a question for you.  What rules do you use to determine if a set of data represents a stable proces?
 
 
Data Set 1: The data is normally distributed and Cpk…[Read more]

Statman:
I confess that I am a litte confused by the statement that it is better than adding a 1.5sigma shift to process A.  Nevertheless, I would say that Montgomery and I would have a different point of frustration (I would love to see the exact pages where Montgomery discuss the issue).  When I examine the difference between Cpk and Ppk, I ju…[Read more]

You, Gabriel, have provided two excellent sets of data.  The two sets of data show why estimating sigma from R_bar/d2 is so dangerous (d2=2.326 for a subgroup of 5).  Estimating s from R_bar/d2 assumes that the distribution is normal.  The following will show that it is better to use the definition of standard deviation, the formula, to ca…[Read more]

Kim:
 I am not sure why Montgomery would say that Ppk is a step backward.  I have several of his books but not the one in question.  I would love to know how Montgomery defines Cpk and Ppk and to see the section in question.   
The fact is, if you calculate Cpk and Ppk using the same set of data, the two values should be very close.  The diffe…[Read more]

John makes an interesting comment.  I never realized that JMP original platform was on the MAC.  I only used a MAC for a short period of time and that is where I first started using JMP, probably version 2.0 or even 1.0.  I used 3.0 on a PC and I am now using 4.0 on a PC.  I have used PCs since the XT, DOS, was around and I do not feel that the…[Read more]

More Clarification! 
 
All the steps in your process appear to be the times required to complete a process step.  That does not produce a one‑side distribution.  The time needed to complete an event is equivalent to the measurement of a physical dimension.  Both measurements, assuming random error, will end up normally distributed.  With time…[Read more]

I use JMP, and I have used it for years. JMP will do everything you are asking for and more.  I have used other programs to a limited extent; so I will comment on JMP only.  I have trained others on using JMP, and this is what I have found.  
I find the program easy to use.  The people that I have trained have found the package easy to use once t…[Read more]

I have been out of the loop on this discussion for roughly a month and changed e-mail address during that time.  Now, I am back and I shall ask two questions. Gabriel, you said, “But with Cpk and Ppk, int’s not just to ways of estimating the same things. You are estimating different things (inherent variation vs total variation), and hence 2 di…[Read more]

Matt:
It has been a while since I have had a chance to follow the discussion.  First, it was pointed out that I made a mistake in the calculation of Cpk using R_bar/d2.  When I went back and checked the results.  I failed to divide by three.  The Cpk and Ppk for a uniform distribution of 100 points was much close in value. 
Nevertheless, I kn…[Read more]

Matt:
What I was suggesting is that we use the term Cpk for all the different calculation methods.  We then specify how we did the calculations.  If you want to calculate sest using R_bar/d2, just tell me how you did the calculation.  If you are my supplier, I may ask you to calculate Cpk using a different method that I could then specify. 
Ev…[Read more]

First you have to determine your subgroup size.  The size of the subgroup is something that you determine before you start the calculation.  It seems to be that a subgroup size of 5 is used a lot. 
 
For a subgroup size of 5 (5 batches with one data point per batch) gives d2=2.33.  If you want a different subgroup size you need to let me know…[Read more]

Jeff:
I use SAS Institutes JMP for my statistical analysis so I do not have a copy of Minitab.  I had someone follow  the path
Stat->Quality Tools->Capability Analysis->Help->See Also->Calculations
They gave me a copy of what they found.  The Sigma within is the standard deviation estimate based on within subgroup variation.  I assume that…[Read more]

If your customer requires Cpk, you have no choice but to do it.  On the other hand, I would still track Ppk.  Ppk uses the calculated s.  When you report Cpk or Ppk you need to report how s is determined.     
 
Let me just emphasize one point in the calculation of Cpk or Ppk.  Let’s assume that you examine the same set of data and you calcul…[Read more]

Ashwin:
 
Let me clarify a statement that I made.  I wanted to say, “If Cpk goes down you need to explain why!  If Cpk goes up, should I get a bonus? Ok forget the bonus.” 
 
I believe that we fundamentally agree.  I do not think that we shall have a flame war.  In addition, I do think very highly of statisticians.  Nevertheless, I still bel…[Read more]

Migs:
 
I do not like the way the meaning of Cpk and Ppk have evolved.  Therefore, I hope I got your attention by saying that “Ppk is junk.” 
 
The definition of Cpk is that it uses an estimate of sigma, and Ppk uses the calculated sigma.  In this definition of Cpk, the term estimate of sigma means sigma=R/d2 where R is the range and d2 is…[Read more]

ETAL:
Statistical indices: Different indices mean different thinks.  Etal said, “…BEFORE yields are impacted? ”  
Cpk:   Cpk does not provide information before yields are impacted.  The purpose of  this statistical index is to provide information regarding the capability of your process.  For example, if you have a Cpk of 0.7 and you have…[Read more]

I avoided comments about the integrity of a company that would fudge their data to make themselves look good when I read Dan’s response.  But Eileen your comment, “…you are already in the crosshairs” was great.  While Bullet Boy was a messenger, and it appears that he will be shot, I wonder if his management will suffer the same puni…[Read more]

Katherine:
Dave’s idea of creating a process map is a good idea, and understanding what the customers wants, QFD, is also a very important tasks.  
In many situations, understanding the process under investigation is reasonable clear from personal experience.  Unfortunately, to give specific suggestions that could be useful in the short term req…[Read more]

Just because you have a small Cpk does not imply that you deliver nonconforming parts.  If you are doing 100% inspection prior to shipment, you should catch most of the nonconforming parts. 
What a small Cpk means is that you are not making as much or any money on the parts that you sell.  The question comes down to this.  What is your Cp?  If it…[Read more]

David:
 
You maybe confusing three points: Cp and Cpk, control charts, and six sigma. 
 
Cp and Cpk: Let me give an example regarding Cp and Cpk.  You have a process whose mean is 5.03, standard deviation, s, is 0.15,USL=10, and LSL=5.  From the example below, you can have a great Cp and a poor Cpk.  What this mean is that your process has v…[Read more]