Accuracy vs. Precision - What is more important for Process Capability

What is More Important in Process Capability?

  • Accuracy

    Votes: 4 40.0%
  • Precision

    Votes: 6 60.0%

  • Total voters
    10
  • Poll closed .
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reynald

Quite Involved in Discussions
#2
Re: Accuracy versus Precision

I was about to say both, but you specified to choose only one. Then i say its precision. My stand is "Variation is the enemy". If my process is predictible/consistent, then being off(in-accurate) is easier to remedy than being centered but unpredictible/inconsistent. :)
 

Statistical Steven

Statistician
Staff member
Super Moderator
#3
Re: Accuracy versus Precision

I tend to agree..but I am finding more and more clients that are working on centering their processes versus reducing variability. I say they are taking the easy way out.
 

harry

Super Moderator
#5
Re: Accuracy versus Precision

I tend to agree..but I am finding more and more clients that are working on centering their processes versus reducing variability. I say they are taking the easy way out.
I think it's the lack of knowledge or rather insufficient knowledge. Over here, most general textbooks on statistics, SPC or management (at the introductory or lower levels) drum into your mind about control limits and the need to keep within limits but is silent on issues like accuracy versus precision.
 
J

Jim Shelor

#6
Re: Accuracy vs. Precision - What is more important for process capability?

What is more important for process capability? An accurate process or a precise process...you can only choose one.
Covers,

I am not sure I agree with you.

Let’s take an example. Results from a Gage R&R

Gage R&R

%Contribution
Source VarComp (of VarComp)
Total Gage R&R 0.09143 7.76
Repeatability 0.03997 3.39
Reproducibility 0.05146 4.37
Operator 0.05146 4.37
Part-To-Part 1.08645 92.24
Total Variation 1.17788 100.00


Now let’s double the gage R&R

Gage R&R

%Contribution
Source VarComp (of VarComp)
Total Gage R&R 0.18286 7.76
Repeatability 0.03997 3.39
Reproducibility 0.05146 4.37
Operator 0.05146 4.37
Part-To-Part 1.08645 92.24
Total Variation 1.26984 100.00

This demonstrates that when we double the contribution from the gage, total variation increases by only 0.09143 units increasing the 6 sigma spread of the process by 0.25 units.

On the other hand, if we double the bias (accuracy) of the gage, the mean moves the entire distance of the change in the bias. Assume bias was 2 units and we double it to 4 units. Then one side of the capability curve moves an additional 2 units toward the specification on that side.

For my money, give me an accurate gage. Precision will take care of itself in the R&R process.

Best regards,

Jim Shelor
PMP, CSSBB
 
#7
Re: Accuracy vs. Precision - What is more important for process capability?

Great description, Jim:

Accuracy has it for me too. If your measurement isn't accurate, then the precision is useless, anyway......
 
B

Benjamin28

#8
Re: Accuracy vs. Precision - What is more important for process capability?

The two go hand in hand of course, but I had to vote for accuracy. What good to have a repeatable low variability process if it is not accurate. Makes me think of the term "consistently wrong".
 

Miner

Forum Moderator
Staff member
Admin
#9
Re: Accuracy vs. Precision - What is more important for process capability?

Accuracy is also more insidious than precision. At least a precision problem make itself well known by poor repeatability. If your accuracy changes, you just change the process to compensate without questioning the gage. At least until the next calibration cycle comes around, and you have to do a risk assessment.
 
#10
Re: Accuracy vs. Precision - What is more important for process capability?

This question reminds me of a story in a Quality book, I read a looong time ago. The story concerned two snipers trying to shoot each other. One sniper got off five rounds, all in the other snipers shoulder, grouped so closely you could cover them with a playing card. The wounded sniper drew a bead on the first and fired, hitting the first sniper in the head - killing him!

The moral of the story? Precision is 'pretty' - accuracy is deadly.....
 
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