Normality of Data Question - Minitab Analysis

Hami812

Engineering Program Mg
#1
I have a data set which I looked at via Minitab v16. I want to find out exactly what type of data I am looking at. When I do the normality test in Minitab, it shows P value as less than .05. According to my training if P=.05 or less the data is not normal gausian distribution. I realize there are many different types of data such as Exponential, Poisson, Gaussian, etc. How do I characterize what type of data seti I am dealing with? When I want to find out I-IM value and use Box-Cox transformation, it works fine on the MR graph, but I want ot explain to someone else I am trying to convince in Senior Management what it is we are really doing with the data conversion and why its not normal in the 1st place.

If you look at any of the data sets attached, they all seem to be non-gaussian if I am reading this correctly. Any suggestion. Did I do something wrong? Should I take absolute value instead of leaving negatives values in the worksheets?
 

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bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#2
Re: Question on Normality

What you have is discrete failure modes that are not directly related. There is no reason to believe that they would be "normal", as: first, they are sorted high to low (by design, that is the Pareto technique), which would give you a skewed distribution, and second, there is no "correct" sorting of this data. Now, for either the total failures over time or the failures over time of any individual cause, you might find normality.
 

Jim Wynne

Leader
Admin
#3
Re: Question on Normality

I have a data set which I looked at via Minitab v16. I want to find out exactly what type of data I am looking at. When I do the normality test in Minitab, it shows P value as less than .05. According to my training if P=.05 or less the data is not normal gausian distribution. I realize there are many different types of data such as Exponential, Poisson, Gaussian, etc. How do I characterize what type of data seti I am dealing with? When I want to find out I-IM value and use Box-Cox transformation, it works fine on the MR graph, but I want ot explain to someone else I am trying to convince in Senior Management what it is we are really doing with the data conversion and why its not normal in the 1st place.

If you look at any of the data sets attached, they all seem to be non-gaussian if I am reading this correctly. Any suggestion. Did I do something wrong? Should I take absolute value instead of leaving negatives values in the worksheets?
What are you trying to learn from the data? It appears to me that you have a lot of cumulative numbers in a lot of different categories, and it might be better to examine each category individually in order to learn something about distributions. Trying to fit aggregate data to a curve might yield unexpected results if the nature of the underlying data sets isn't understood.
 

Bev D

Heretical Statistician
Leader
Super Moderator
#4
agree with Bob and Jim: it looks like you are trying to do something with this data that shouldn't be done.

In other words either your statistical analysis isn't appropriate for the question you are really tring to answer, or your data set isn't appropriate for the question you are really trying to answer.

it looks like you may be trying to see if Mexico has a different failure rate than USA?
 
N

NumberCruncher

#5
Hi Hami812

What exactly are you trying to ascertain from your data?

It looks like you are trying to compare failure rates and decide which country has the greatest number of failures.

If I have interpreted your data correctly, for each country:
(total pass + total failure) = total parts tested

If this is the case then you are fundamentally doing the wrong analysis in simply comparing numbers. You don't always have the same number of tested items in each category for each country.

For example, HDD test for both USA and Mexico, the total number of items is 606.
However, for Boot Software Upgrade, USA = 1200 items, Mexico = 1176.

Comparing absolute numbers becomes less and less meaningful the greater the difference in the number of items tested.

For the example of Boot Software Upgrade, if you had a 50% failure rate for both countries, USA would have 600 fails but Mexico would have 588 fails. So the USA has a greater number of fails. Except that it doesn't. The percentage failure rate is the same.

If this is what you are trying to do, I would suggest comparing % failure rate in each category rather than comparing absolute numbers, and forget about normality of the data.

NC
 
W

WKHANNA

#6
The question I would ask, taking for granted a great deal of the OP’s specific issue, is if there is MSA data on the test equipment used at both locations. Not to mention calibration & training records.
 

Hami812

Engineering Program Mg
#7
agree with Bob and Jim: it looks like you are trying to do something with this data that shouldn't be done.

In other words either your statistical analysis isn't appropriate for the question you are really tring to answer, or your data set isn't appropriate for the question you are really trying to answer.

it looks like you may be trying to see if Mexico has a different failure rate than USA?
NumberCruncher,

An Attribute Analysis was already performed on the differences between Mex and US in a previous worksheet used to compare Passes with Failures between 2 sites using the same tools. Between appraisers showed in this Gage R&R (attribute study only) an 84% agreement with US for 2 out of 3 benchs when 200 units of product were tested at 3 benchs (2 times for each unit product) for a total of 1200 tests. the 3rd bench showed a 97% agreement with US. My question was how I can build the case to begin using Statistical Process Control by starting with Attribute Analysis, showing there is a delta between and within appraisers being less than 90% (cust expects 95% agreement between and within appraisers). So first I was trying to show them there is a mismatch, next step was to show them that in order to keep both locaiton in check from a quality standpoint, some SPC methods need to be put into place. There are none today other than occassion gage R&R for Attibute Analysis which I put into place Quarterly. So my quesiton is more me trying to have a broader understanding of the nature of the data itself in order to convince the SMT that putting SPC in place is a good idea for both locations. I want them to adopt the use of Control Charting tools at both sites and I beleive I will get a lot of blockers at Senior level of my company (status quo changes are bit tricky here). So I guess I am coming at this a bit strange, but I need to understand the nature of data a bit more. My goal is to get process control into place here. This COmpany I work for has TL9000 but no Quality control methods for changing process or moinitoring techniques in place other than dollars which is usually far too late to know when things have gotten out of control. I think what i need to do is show them their proces is not in control and using control charts requires the data analysis to be Gaussian in nature. However, the Control Charting in Minitab allow any of the data to be converted using Box-Cox. But I need to understand when I should use Box-Cox to transform the data. I am not 100% comfortable with knowing if and when it needs to be converted from X type of data to "Normal" distribution. I think this was the crux of my quesiton.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#8
At this point, all you have provided to us is attribute data. None of the data represents failure over time, as you would have in a capability study. That variation is what you want to see.

But, in my mind, SPC with the attributes you identified will only be a report card chart - it will represent that "whatever you did" works or not. It is one use...but one of the weakest uses of control charts.

I suggest using this pareto chart to pick out your worst case. Then determine what causes allow the failure to happen. Are any of the causes variable data? Can they be controlled (dialed in?). If so, do a capability study of that variable, determine its distribution, develop an appropriate charting methodology. Then...go on to the next one.

Using that approach, it will be much more easy to convince management of the value of SPC (at least it would impress me a whole lot more).
 

Bev D

Heretical Statistician
Leader
Super Moderator
#9
Also, if you are going to place each defect type on a control chart (and yes you are ready to go do that) you will not have to 'transform' the data so it is Normal. just use the appropriate chart type for your data. in this case I'd start by trying a u chart. you may need or want to go to an I, MR chart, but start with the u. Control charts do not require Normality (Gaussian data) to work. That is a myth. Control charts do require rational subgrouping and a most chart types require a homogenous process stream, although some charts do incorporate adjustments for systemic causes that are not removable.

Try plotting the run charts of some of your defects and we can take a look to help you ensure that you have the correct charts.
 
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