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