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Capability analysis - What is going on this chart?

reunalla

Involved In Discussions
#1
Hi,
can somebody tell me what is going on this chart? (see attached pic)
I am very bad at statistics.

I do not think this process is capable. Is it capable? Is it stable?

Thanks in advance Capab.jpg
 
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Bev D

Heretical Statistician
Staff member
Super Moderator
#2
With only the chart to go by, it is not stable, but it does look capable. You have out of control points (outside the upper control limit) and so the process is not stable. The upper limit (which is the only one that matter in the parallelism measurement, as zero is perfection) is very far from even the highest value. So even tho it shows as an unstable process I would say that it is capable as the instability is so small.

These studies are so often more about the physics than the ‘blind’ application of some statistical test, even as robust a test as SPC or as flawed a test as Cpk.

Is there value in understanding what the ‘out of control’ high values are? To do that we would need to understand more about the process teh part and measurement system.
 

Proud Liberal

Quite Involved in Discussions
#3
Your Cp and Ppk values aren't being calculated correctly because you entered a zero as the LSL and Minitab is working on the assumption that negative values are possible (which they aren't). Leave that field blank to force a bounded tolerance calculation.

Also, if you retained your original samples, you might want to re-check your flier points. My first thought is the cleanliness of the part or surface plate.
 

Miner

Forum Moderator
Staff member
Admin
#4
Your Cp and Ppk values aren't being calculated correctly because you entered a zero as the LSL and Minitab is working on the assumption that negative values are possible (which they aren't). Leave that field blank to force a bounded tolerance calculation.
Check the Boundary box (not available in the capability six pack).
1588598707586.png

Better yet, use the non-normal capability analysis. You will need to fit a distribution first. Lognormal should be close.

1588598829297.png
 
Last edited:

Bev D

Heretical Statistician
Staff member
Super Moderator
#7
Just some advice: Capability studies aren't about how to crank out some stats and graphs in minitab. It really is about understanding your process and it's actual capability. How involved are you in the actual study design, the measurement process and the process itself?
 

reunalla

Involved In Discussions
#8
Can you attach your data?
Yes, in the attachment.

Just some advice: Capability studies aren't about how to crank out some stats and graphs in minitab. It really is about understanding your process and it's actual capability. How involved are you in the actual study design, the measurement process and the process itself?
I am not involved, fortunately. I am working as a Quality Engineer/Consultant but I tell everybody that I do not understand this and it is not part of my job. But it came up at a work discussion and I am trying the understand. Also, I am currently at uni studying metrology and we are learning statistics and so to understand it better will help on the finals. We will have some classes this week about Minitab (online class due to COVID-19).

Check the Boundary box (not available in the capability six pack).

Better yet, use the non-normal capability analysis. You will need to fit a distribution first. Lognormal should be close.

View attachment 26790
I tried this one but it says that the lower spec cannot be 0 :(
 

Attachments

Miner

Forum Moderator
Staff member
Admin
#9
As I review your data, I see a possibility of a mixture of different process streams. Can you provide some detail on the process and how the data were collected?
 

Welshwizard

Involved In Discussions
#10
Hi Reunalla,

Looking at your initial questions:

"I do not think this process is capable. Is it capable? Is it stable"

Its very common for people to ask about capability of a data set initially but in order to answer this question with any degree of confidence we first have to answer the question of stability i.e. consistency in the behaviour of the data.

In order to understand stability we must have knowledge about the context of the data. Remember, no data has meaning without context, in this instance and in keeping with what has already been iterated in this thread what does the data represent, how was it gathered, who gathered it, under what conditions was it gathered, what was the method of collection etc etc.

The data as shown does not appear stable so any question of stating the capability is moot, however, the data may contain many sources of variation which may render it unsuitable to assess stability,consider this as the process trying to tell you that all is not well and therefore that you must investigate.

In summary, check the context, check for stability, if its not stable (basic rules for a control chart) find out why and fix then read off the capability indexes. If you really have to accept data like this then you state that its not stable (as shown) and the hypothetical capability is X or Y. Looking at your data there is a reasonable hypothetical chance that once the process is stable you will have some reasonable capability but the foundation to the data set has multiple personalities which can change in an instant, which one are you going to listen to?

Hope this helps

Good luck
 
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