Process Capability
DS-624 Quality Management
Chapter 14
E. Measurement System Analysis
DS-624 Quality Management
What is Measurement System Analysis (MSA)?
Why is it important to analyze the measurement system?
Schematically describe overall variation
What is Gage R&R?
What is Repeatability?
What is Reproducibility?
What are the common errors made while performing GR&R:
Measurement System Analysis
What is Measurement System Analysis (MSA)?
It is the process of measuring one or more of these measurement systems characteristics:
Calibration: Drift in average measurements of an absolute value.
Stability: Drift in absolute value over time.
Repeatability: Variation in measurement when measured by one appraiser on the same equipment in the same measurement setting at the same time.
Reproducibility: Variation in measurement when measured by two or more appraisers multiple times.
Linearity: Accuracy of measurement at various measurement points of measuring range in the equipment.
Bias: Bias (difference between absolute value and true value) with respect to a standard master at various measurement points of the measuring range.
Accuracy: Closeness to the true value, or to an accepted reference value.
Precision. Closeness of repeated readings to each other. A random error component of the measurement system.
Measurement System Analysis
Why is it important to analyze the measurement system?
We introduce errors of the data during the process of collecting the data. It is important t understand how much error is being introduce before moving forward with further analysis with flawed data.
In the quest to reduce variation the measurement system should be one of the first things analyzed because all data from the process are, in effect, filtered through the measurement system.
It is not uncommon for measurement systems to have an error of 40 to 50 percent of the process specification.
Overall Variation
Schematically describe overall variation
Repeatability & Reproducibility
What is Gage R&R?
Methodology used to define the amount of variation in the measurement data due to the measurement system.
It compares measurement variation to the total variability observed, consequently defining the capability of the measurement system.
Measurement variation consists of two important factors, repeatability and reproducibility.
Repeatability & Reproducibility
What is Repeatability?
Is the equipment measurement variation expressed as standard deviation. Measurements are taken from the same equipment by one appraiser over a short period of time.
The repeatability portion of the measurement variation is attributed to the inherent variation of the measurement equipment.
It is often referred to as equipment variation.
Repeatability & Reproducibility
What is Reproducibility?
Is the appraiser measurement variation expressed as standard deviation. Measurements are taken from the same equipment by more than one appraiser.
In the case of reproducibility, the influential factors are the setting of the work piece (any special loading and unloading), operator training, skill, and knowledge, consistency in measurement, and so on.
Gage Repeatability & Reproducibility
What are the common errors made while performing GR&R:
In process control situations, not selecting samples covering the tolerance spread.
Not randomizing the samples during measurement.
Using untrained appraisers or process-unrelated employees in the experiment because there are not enough appraisers to conduct the studies.
Altering the samples during the study.
Experimenter not present during the R&R study.
Publishing the results with the appraisers’ names.
Assuming that the GR&R results are valid forever.
Assuming that GR&R performed on a specific piece of equipment is the same as for all other equipment of that kind.
