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%