Measurement System Analysis
Measurement System Analysis (MSA) is an experimental and mathematical method of determining how much the variation within the measurement process contributes to overall process variability.
A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce.
A measurement systems analysis evaluates the test method, measuring instruments, and the entire process of obtaining measurements to ensure the integrity of data used for analysis (usually quality analysis) and to understand the implications of measurement error for decisions made about a product or process.
The purpose of Measurement and Analysis (MA) is to develop and sustain a measurement capability used to support management information needs. When data are shared widely across projects, data may reside in the organization's measurement repository.
Systems of measurement in use include the International System of Units (SI), the modern form of the metric system, the imperial system, and United States customary units.
A measurement system can be characterized, or described, in five ways
Location (Average Measurement Value vs. Actual Value):
- Stability refers to the capacity of a measurement system to produce the same values over time when measuring the same sample. As with statistical process control charts, stability means the absence of "Special Cause Variation", leaving only "Common Cause Variation" (random variation).
- Bias, also referred to as Accuracy, is a measure of the distance between the average value of the measurements and the "True" or "Actual" value of the sample or part. See the illustration below for further explanation.
- Linearity is a measure of the consistency of Bias over the range of the measurement device. For example, if a bathroom scale is under by 1.0 pound when measuring a 150 pound person, but is off by 5.0 pounds when measuring a 200 pound person, the scale Bias is non-linear in the sense that the degree of Bias changes over the range of use.
Variation (Spread of Measurement Values - Precision):
- Repeatability assesses whether the same appraiser can measure the same part/sample multiple times with the same measurement device and get the same value.
- Reproducibility assesses whether different appraisers can measure the same part/sample with the same measurement device and get the same value.