Specification Limits and Capability Studies in Minitab

S

sunff114

#1
My understanding about the specification limits is the following.

If the product does not meet the specification limits, then the product is nonconformity. right?

specification limits are different with control limits.
 
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K

Ka Pilo

#2
Re: Specification limits

From Wikipedia:

Control limits, also known as natural process limits, are horizontal lines drawn on a statistical process control chart, usually at a distance of ±3 standard deviations of the plotted statistic from the statistic's mean.
Control limits should not be confused with tolerance limits or specifications, which are completely independent of the distribution of the plotted sample statistic. Control limits describe what a process is capable of producing (sometimes referred to as the “voice of the process”), while tolerances and specifications describe how the product should perform to meet the customer's expectations (referred to as the “voice of the customer”).
From Businessdictionary:
Specification limits - Boundaries or parameters that define acceptable performance for a process expressed as a target limit as well as an upper and lower limit.
The dictionary gives numerous definitions to the same word.
 
S

sunff114

#3
Re: Specification limits

I know the difference between UCL/LCL and USL/LSL. I am conducting capability analysis using Minitab, but I am not sure about how to choose the specification limits.

The only information I can get from the original report sheet is that the standard value has to be greater than 3.5. I am not sure if I should pick 3.5 as LSL only, or I can use UCL and LCL as the USL and LSL, respectively.
 

Marc

Fully vaccinated are you?
Leader
#4
Re: Specification Limits - When a product does not meet the specification limits

My understanding about the specification limits is the following.

If the product does not meet the specification limits, then the product is nonconformity. right? <snip>
Correct.

I know the difference between UCL/LCL and USL/LSL. I am conducting capability analysis using Minitab, but I am not sure about how to choose the specification limits.

The only information I can get from the original report sheet is that the standard value has to be greater than 3.5. I am not sure if I should pick 3.5 as LSL only, or I can use UCL and LCL as the USL and LSL, respectively.
You have two questions here, I think. First is: "...If the product does not meet the specification limits, then the product is nonconformity. right?...". Second is you ask about how you "choose" specification "limits". Specifications are set during the design phase.

Can you be a bit more specific about what you mean by "...choosing the specification limits"?

Maybe I'm missing something here.
 
T

The Specialist

#5
Re: Specification Limits - When a product does not meet the specification limits

Correct.

You have two questions here, I think. First is: "...If the product does not meet the specification limits, then the product is nonconformity. right?...". Second is you ask about how you "choose" specification "limits". Specifications are set during the design phase.

Can you be a bit more specific about what you mean by "...choosing the specification limits"?

Maybe I'm missing something here.

I think that the OP means; How do I determine the specification limits.

RA/FMEA etc...
 

Marc

Fully vaccinated are you?
Leader
#6
Re: Specification Limits - When a product does not meet the specification limits

I think that the OP means; How do I determine the specification limits.

RA/FMEA etc...
We won't know until the OP replies.

Determine vs. Choosing are pretty much the same thing. I think Defining would also be appropriately descriptive.

Question is, when the OP asks/says: "I am conducting capability analysis using Minitab, but I am not sure about how to choose the specification limits." What is the OP actually asking? If the OP is doing a capability analysis the specification limits should be a known value defined in the print or relevant document.

Again, I may be missing something but that's my understanding.
 
T

The Specialist

#7
Re: Specification Limits - When a product does not meet the specification limits

We won't know until the OP replies.

Determine vs. Choosing are pretty much the same thing. I think Defining would also be appropriately descriptive.

Question is, when the OP asks/says: "I am conducting capability analysis using Minitab, but I am not sure about how to choose the specification limits." What is the OP actually asking? If the OP is doing a capability analysis the specification limits should be a known value defined in the print or relevant document.

Again, I may be missing something but that's my understanding.

Sorry Marc... My post may have come off wrong!

The way I read it, 'choosing' came across as something you do without study/consideration and assessment of requirements.

You are right...

I will leave it to the OP to confirm.

(Just trying to help clarify!)

Oh... I agree with your confusion, re specification limits being a known value. This is what lead me to assume that the OP has not yet determined the required specification/tollerances.

Back under my rock, I think!!
 
S

sunff114

#9
Re: Specification Limits - When a product does not meet the specification limits

Thank you all for the concerns to this topic!

let me expain my problems.
To conduct the capability analysis (assume the process output is stable and normally distributed), we have to know the USL or LSL. I didn't observe the process of data collection. I only have the final data report sheet. On that sheet, it said that standard value of this product is greater than 3.5. (i.e. min = 3.5) and the target value is 4.59. That is, a nonconformity will be considered if any value of this product is less than 3.5.

I am thinking about two approaches for the specification limits.

One is that the LSL should be 3.5, and NO USL.

The other is to use the UCL and LCL as the USL and LSL, respectively.

I tried both approaches in minitab, and it is obvious that the results are dramatically different.

My question is which approach is the right way to set up the specification limit.

Please let me know if there is any better method. Thanks again!
 
B

Barbara B

#10
Re: Specification Limits - When a product does not meet the specification limits

To conduct a capability analysis in Minitab, you don't need any control limit(s), you only need specification limit(s).

If the final report is somewhat loosely defined, why not ask someone which knows the process and requirements (e.g. the customer)? 3.5 could be a process limit (LCL) or a specification limit (LSL). My guess would be specification limit, but despite some widespread rumors statistics are not a matter of tea leaves reading but rather some kind of science :bigwave:

Regards,

Barbara
 
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