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Assessing components of Measurement systems (MSA)

by pmgindia

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Measurement System Analysis (MSA) is designed experiment that seeks to identify the components of variation in the measurement. If measurements are used to guide decisions, then it follows logically that the more error there is in the measurements, the more error there will be in the decisions based on those measurements. The purpose of MSA is to qualify a measurement system for use by quantifying its accuracy, precision, and stability. Quality System Standards such as QS 9000/ ISO/ TS/ 16949 are now mandatory for automobile industries & Suppliers to automobile industries. As per these standards the analysis of measuring process variations is required & they also demand ensuring of control on monitoring & measuring devices. Finally, it helps to have proper control on the Overall Process, by way of Measurement System Analysis (MSA). The measurement systems analysis process is defined in a number of manuals like failure mode and effects analysis (FMEA) and Control Plan manual, statistical process control (SPC) manual, measurement systems analysis manual, production part approval process (PPAP) manual. The purpose of measurement is to provide information about a quantity of interest.

Components of MSA:

1.)    Location variation

  • Bias: It is the difference between the observed average of measurement and the reference value. The reference value, also known as the accepted reference value or master value, is a value that serves as an agreed-upon reference for the measured values. A reference value can be determined by averaging several measurements with a higher level of measuring equipment.
  • Stability:  It is the total variation in the measurements obtained with a measurement system.


2.)    Width Variation

  • Linearity: It is the difference in the bias values through the expected operating range of the gauge.
  • Repeatability & Reproducibility:  Repeatability is is the variation in measurements obtained by one appraiser, with one measurement instrument, when used several times, while measuring the identical characteristic, on the same part. And Reproducibility is is the variation in the average of the measurements made by different appraisers, using the same measuring instrument, when measuring the, identical characteristic, on the same part.

If repeatability is large compared to reproducibility, the reasons may be the instrument needs maintenance;the gauge should be redesigned to be more rigid; the clamping or location for gauging needs to be improved; there is excessive part variation.

If reproducibility is large compared to repeatability, then possible causes could be: the appraiser needs to be better trained in how to use and read the gauge instrument; calibrations on the gauge dial are not clear; a fixture of some sort may be needed to help the appraiser use the gauge more consistently.

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 and to understand the implications of measurement error for decisions made about a product or process. MSA is also an important element of Six Sigma and of other quality systems management.<a href="">MSA</a>

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