About this lesson
These three attributes of a measurement system are inherent in the design and management of the system. When not managed well they will prevent effective measurements. Each of these is discussed and principles for managing them are introduced.
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Stability, Linearity, Discrimination
Stability, linearity, and discrimination are categories of measurement system error that are based on the design and maintenance of the measurement system. These errors can be reduced and eliminated by the selection of the measurement system to be used.
When to use
Stability, linearity, and discrimination are aspects of the measurement system that should be addressed when designing or selecting a measurement system for use. Once the system is selected, linearity and discrimination cannot be changed and as long as the measurement system is managed well, stability should not be an issue.
Stability, linearity, and discrimination are normally aspects of the measurement system that are outside the control of the operator. These are inherent in the design of the selected system and must be monitored or controlled by the management of the measurement process.
- Stability is the most volatile of these three. Stability of a measurement system is the extent to which the measurement distribution stays the same over time. A stable system does not drift or deteriorate. The stability of the system is significantly impacted by how the system is managed. Many measurement systems use regularly scheduled calibration and encourage the use of golden standards by operators to identify stability problems when they are small and can be easily corrected. There are three categories for causes in of instability in measurement systems:
- Standard Operating Procedures – Well designed procedures that are followed by all operators will minimize stability errors due to the operators modifying the measurement process over time. Operators should be trained and periodic certification or audits can be used to ensure continued compliance.
- Noise factors – these are aspects of measurement system use that cause distraction or confusion. These factors are often related to the environment in which the measurement system is used. If the environment changes over time, these factors may take on a larger or smaller role in the normal distribution of the measurement system error. The selection of the measurement system can minimize these factors by using systems that have Poka Yoke features to reduce variation.
- Controllable factors – these are factors that the operators or manager can monitor and control. An effective procedure and trained operators will monitor these and take the appropriate action to ensure that the target settings or controls are maintained. This may mean adjusting settings for changes in environmental factors, monitoring the system for wear and tear, and the use of golden standards to detect when the system performance is starting to change.
- Linearity is also known as bias shift. This is an attribute of the design of the measurement system and normally cannot be changed. This refers to the condition that the measurement system bias begins to change once it is near the edge of its ideal operating range. As the system attempts to measure very small or very large items, the performance degrades and the bias shifts. This is common in electronic systems with sensors or systems that rely on springs. In both cases there is an ideal range of performance. If the system is not linear throughout the normal range of measurement, a different system should be selected.
- Discrimination refers to the resolution of the system metric. A system with poor discrimination is not able to measure small differences in the items being measured. Discrimination is affected by the desired precision in the measurement. The measurement system should always be able to go one decimal further in resolution than the required specification value. Ideally a measurement system has a minimum of 10 gradations in the normal range of item variation – and more is definitely better. Poor discrimination leads to a uniform distribution which cannot be used with many of the statistical analyses done in Lean Six Sigma projects. If the measurement system does not have adequate discrimination, a different system must be selected.
Hints & tips
- Determine the measurement needs in terms of the range or measurement and the fidelity or precision of the measurement. Then select or design a system that has adequate discrimination and is not subject to linearity problems.
- Many organizations have a metrology department (or at least an individual who is responsible for metrology) that manages and monitors measurement system stability. This department establishes a calibration schedule for each measurement system and creates and maintains any golden standard that operators are to use as part of their procedures.
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