Measurement System Evaluation pp

Needed because total variance of process recorded is the sum of process variation and measurement variation Thus, when there is a special cause, always look first at measurment tool 2

Metrology: the science of measurement Goal: zero error in measurement –accurate: difference between true value and observed –Precision: variability of accuracy –Repeatability: variation when same instrument is used by same operator, this variance is often called equipment variation –Reproducible: is the same accuracy and precision seen across all operators 3

Low Technology calibers tapes observation, count thermometer scales 4

High Technology lasers optics microprocessors sound chemical 5

Evaluation and verification of measurement Select m operators and n parts (samples), minimum of two operators and 10 parts calibrate measuring instrument against standard have each operator measure each part in random order for a minimum of 2 trials Compute means and ranges EV, AV R&R and these as percent of tolerance range, see excel folder 'Measurement Evaluation Example' 6

Calibration Required if selling based on a measure (weight, size, concentration) Measuring instrument has to be verified by independent entity, usually government and/or industry Have to confirm calibration throughout time (self and independent entity) Evaluation of equipment (is it sound) What tolerances are needed by device What tolerances are allowable (specifications) How to calibrate How often does device need tested Random testing schedule Documentation and reporting system Independent evaluator doing their schedule 7