MINITAB and non-normal unilateral tolerance data - Cannot confirm Ppk values

[VMax!]

Hi folks! First time poster here.

One of our suppliers recently FAXed over some parallelism data (using DataMyte/Quantum SPC). I manually copied the raw data over into MINITAB (our software package) and found (not surprisingly) the data is not normal (A^2 = 6.95, N=0.000). I was taught that if the data was not normal I should not do any further statistical analysis.

I have no reason to believe their Quantum data is invalid, but I cannot confirm their Ppk values using MINITAB. What to do?

Thanks in advance for your thoughts.
Kevin

[Miner]

In Reply to Parent Post by VMax!

Hi folks! First time poster here.

One of our suppliers recently FAXed over some parallelism data (using DataMyte/Quantum SPC). I manually copied the raw data over into MINITAB (our software package) and found (not surprisingly) the data is not normal (A^2 = 6.95, N=0.000). I was taught that if the data was not normal I should not do any further statistical analysis.

I have no reason to believe their Quantum data is invalid, but I cannot confirm their Ppk values using MINITAB. What to do?

Thanks in advance for your thoughts.
Kevin

The old "If the data are not normal..." is not valid. However, you do have to be careful to use the correct statistical techniques.

Minitab has an option to analyze non-normal data either by distribution fitting or by data transforms (to normalize the dats). Note: it is perfectly "Normal" for parallelism data to be non-normal. You are dealing with what is called a "Boundary" condition (i.e., you cannot go below zero). Minitab can hadle this also. Enter 0 as the lower spec and check the Boundary box.

[pabloquintana]

Man, first you can try to identify which distribution fits best using Quality Tools/Individual Distribution Identification.

After that you can use the Non-Normal Capability analysis selecting the distribution you've found in the previous step.

Also you can use a Box Cox transformation in Normal Capability and using 0 as the lower spec and checking Boundary.

Good luck.

[VMax!]

Regarding 'boundary' you both are correct. I checked the box and re-ran MINITAB and I now agree with my supplier's Ppk. Thanks guys.

Regarding 'normality' I am still, well, unsure. In this instance I understand parallelism is a unilateral tolerance (can't be less than zero). So...

1. Should I just 'accept' the fact that the data is non-normal and predict Ppk?

2. Are there other MINTAB functions that I can apply to a given set of 'non-normal' data?

My first inclination when viewing the data was, 'ah, they're cherry-picking their points.' All opinions, URL's, enlightenment, or gifts of money would be appreciated.

[pabloquintana]

In Reply to Parent Post by VMax!

1. Should I just 'accept' the fact that the data is non-normal and predict Ppk?

2. Are there other MINTAB functions that I can apply to a given set of 'non-normal' data?

VMax, you can't apply regular capability analysis to non-normal data. You will have to transform it with Box Cox or identify which distribution matches best to then use the Quality Tools/Non-Normal Capability Analysis of MINITAB.

If you use regular capability analysis to non-normal data you'll get the wrong information.

I will recommend www.isixsigma.com forums for these topics. Do a search or ask there and you'll get a ton of replies in minutes.

Regards.

[Darius]

Many software including MINITAB obtain the ppk no matter if "is normal" or not.

The first step to compare results, check if MINITAB obtain the same value without any transformation (I am pretty sure that the results will be equal), and if not is equal, try first to see if changing options in the configuration screens make them equal (A friend of my obtained two different values in 2 differents reports of MINITAB and checking the options changed both to the same value).

Do not use a lower spec of 0 if is not the case, the indicators like ppk take in account that the target is centered on the specs.

Agree that a transformation (and I really like Box Cox) could transform your distribution an the value obtained will represent with more reality the data but as I think "If you are going to jump into a pool is better to try to see the bottom of the pool before jumping than not doing it because you have no rule to measure it.". An Indicator by it self is better than none.

note: if you are going to transform the data, you will have to transform the specs also.

[pabloquintana]

In Reply to Parent Post by Darius

1. Many software including MINITAB obtain the ppk no matter if "is normal" or not.

2. Do not use a lower spec of 0 if is not the case, the indicators like ppk take in account that the target is centered on the specs.

3. note: if you are going to transform the data, you will have to transform the specs also.

Darius,

I will comment what you've just said.

1. Yes, MINITAB will output a Ppk result no matter what distribution you input but YOU WILL GET THE WRONG INFO!

2. He said that he is using a set of data from a parallelism test. That will have a boundary of 0.

3. The specs will be automatically transformed by MINITAB. Maybe you meant that the result will show transformed specs, but you will be looking at the Ppk values only.

Regards.

[KenK]

A few additional thoughts:

1. Make sure you are using the latest version of MINITAB, which is MINITAB 14.20. Release 14 has a much much more powerful set of non-normal tools than earlier versions, such as release 13.

2. The capability tools do handle one-sided distributions. Just leave the non-applicable spec limit blank. Only check the boundary checkbox if it is physically impossible for a value to fall below or above the boundary. The boundary checkbox is used when the PPM values are calculated, excluding the distribution on the other side of the boundary from PPM calculations.

3. As mentioned several times before, make sure you use the Individual Distribution Identification tool to find the distribution that fits your data best. If that doesn't work well, try the Johnson Transformation option inside the Nonnormal Capability tools.

4. Don't forget that MINITAB users can always call Minitab Technical Support. Except for the cost of the call, it is free to all users. For the number go to Help > About MINITAB and find the Contact Us button. They are very helpful.