Measurement System Analysis: Hidden Factory Evaluation
What Comprises the Hidden Factory in a Process/Production Area?
- Reprocessed and Scrap materials — First time out of spec, not reworkable
- Over-processed materials — Run higher than target with higher
than needed utilities or reagents - Over-analyzed materials — High Capability, but multiple in-process
samples are run, improper SPC leading to over-control
What Comprises the Hidden Factory in a Laboratory Setting?
- Incapable Measurement Systems — purchased, but are unusable
due to high repeatability variation and poor discrimination - Repetitive Analysis — Test that runs with repeats to improve known
variation or to unsuccessfully deal with overwhelming sampling issues - Laboratory “Noise” Issues — Lab Tech to Lab Tech Variation, Shift to
Shift Variation, Machine to Machine Variation, Lab to Lab Variation
Hidden factory Linkage –
- Production Environments generally rely upon in-process sampling for adjustment
- As Processes attain Six Sigma performance they begin to rely less on sampling and more upon leveraging the few influential X variables
- The few influential X variables are determined largely through multi-vari studies and Design of Experimentation (DOE)
- Good multi-vari and DOE results are based upon acceptable measurement analysis
Measurement System Terminology
Discrimination – Smallest detectable increment between two measured values
Accuracy related terms
True value – Theoretically correct value
Bias – Difference between the average value of all measurements of a sample and the true value for that sample
Precision related terms
Repeatability – Variability inherent in the measurement system under constant conditions
Reproducibility – Variability among measurements made under different conditions (e.g. different operators, measuring devices, etc
Stability – distribution of measurements that remains constant and predictable over time for both the mean and standard deviation
Linearity – A measure of any change in accuracy or precision over the range of instrument capability
Measurement System Capability Index – Precision to Tolerance Ratio:
- P/T = [5.15* Sigma (MS)]/Tolerence
- Addresses what percent of the tolerance is taken up by measurement error
- Includes both repeatability and reproducibility: Operator * Unit * Trial experiment
- Best case: 10% Acceptable: 30%
Note: 5.15 standard deviations accounts for 99% of Measurement System (MS) variation. The use of 5.15 is an industry standard.
Measurement System Capability Index – %Gage R & R:
- % R & R =[Sigma (MS)/Sigma(Observed Process Variation)]*100
- Addresses what percent of the Observed Process Variation is taken up by measurement error
- %R&R is the best estimate of the effect of measurement systems on the validity of process improvement studies (DOE)
- Includes both repeatability and reproducibility
- As a target, look for %R&R < 30%