Measurement System Analysis (4TH EDITION) Updated

MEASUREMENT SYSTEMS ANALYSIS

Introduction: - Measurement data are used more often and in more ways than ever before. For instance, the decision to adjust a manufacturing process is now commonly based on measurement data. The data, or some statistic calculated from them, are compared with statistical control limits for the process, and if the comparison indicates that the process is out of statistical control, then an adjustment of some kind is made. Otherwise, the process is allowed to run without adjustment. Another use of measurement data is to determine if a significant relationship exists between two or more variables. For example, it may be suspected that a critical dimension on a molded plastic part is related to the temperature of the feed material. That possible relationship could be studied by using a statistical procedure called regression analysis to compare measurements of the critical dimension with measurements of the temperature of the feed material.

The benefit of using a data-based procedure is largely determined by the quality of the measurement data used. If the data quality is low, the benefit of the procedure is likely to be low. Similarly, if the quality of the data is high, the benefit is likely to be high also. 

To ensure that the benefit derived from using measurement data is great enough to warrant the cost of obtaining it, attention needs to be focused on the quality of the data.

Purpose: - The purpose of this document is to present guidelines for assessing the quality of a measurement system. Although the guidelines are general enough to be used for any measurement system, they are intended primarily for the measurement systems used in the industrial world.

Terminology: - This section provides a summary of such terms which are used in this manual. In this document, the following terms are used:

• Measurement is defined as “the assignment of numbers [or values] to material things to represent the relations among them with respect to particular properties.” This definition was first given by C. Eisenhart (1963). The process of assigning the numbers is defined as the measurement process, and the value assigned is defined as the measurement value.

• Gage is any device used to obtain measurements; frequently used to refer specifically to the devices used on the shop floor; includes go/no-go devices.

• Measurement System is the collection of instruments or gages, standards, operations, methods, fixtures, software, personnel, environment and assumptions used to quantify a unit of measure or fix assessment to the feature characteristic being measured; the complete process used to obtain measurements.

Summary of Terms  Standard 

• Accepted basis for comparison 
• Criteria for acceptance 
• Known value, within stated limits of uncertainty, accepted as a true value 
• Reference value: - A standard should be an operational definition: a definition which will yield the same results when applied by the supplier or customer, with the same meaning yesterday, today, and tomorrow

Basic equipment 

• Discrimination, readability, resolution 

 Alias: smallest readable unit, measurement resolution, scale 

limit, or detection limit  An inherent property fixed by design  Smallest scale unit of measure or output for an instrument  Always reported as a unit of measure  10 to 1 rule of thumb 

• Effective resolution 

 The sensitivity of a measurement system to process variation for 

a particular application 

 Smallest input that results in a usable output signal of 

measurement  Always reported as a unit of measure 

Reference value 

 Accepted value of an artifact  Requires an operational definition  Used as the surrogate for the true value 

• True value 

 Actual value of an artifact  Unknown and unknowable

Location variation 

• Accuracy 

 “Closeness” to the true value, or to an accepted reference value  ASTM includes the effect of location and width errors 

• Bias 

 Difference between the observed average of measurements and 

the reference value  A systematic error component of the measurement system 

• Stability 

 The change in bias over time  A stable measurement process is in statistical control with 

respect to location  Alias: Drift 

• Linearity 

 The change in bias over the normal operating range  The correlation of multiple and independent bias errors over the 

operating range  A systematic error component of the measurement system.

Continue Ahead 

will soon updated with FMEA (Failure Mode of Effective Analysis)