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## Quick reference

### Measurement Systems

The collection of data assumes that there is a measurement system used to measure and record the data. It is important to understand the characteristics of the measurement systems so that the Lean Six Sigma team will know how far they can trust the data.

### When to use

The measurement system is needed when collecting data. It is first used in the Measure phase. If major changes occur in the process, the measurement system may need to be re-evaluated in the Improve phase.

### Instructions

Whenever data is collected, the measurement system must be considered. Measuring is a process, and like all processes it has an inherent level of variation. This variation can create errors in the recorded measurement. When the measurement system error is very small relative to the variation in what is being measured, the recorded measurement will be extremely close to the true measurement. When the measurement system error is very large relative to the variation of what is being measured, the recorded measurement is unlikely to be accurate.

Incorrectly recorded measurements can lead to wrong decisions about the acceptability of process outputs, the stability of the process, and the types of improvements needed in the process. A measurement system study should be done when recording process data to be sure the data is accurate. If the measurement error is too large, the measurement system must be changed before process data is collected. The techniques for conducting a measurement system analysis are taught in another course in this overall series on Lean Six Sigma.

Mathematically, the contribution of the measurement system error to the recorded measurement is reflected in the diagram and equations shown below.

### Hints & tips

- Your measurement system may be a visual check by a process operator. That is still a measurement system and is subject to errors – one operator may say something is good and other say the same item is defective.
- If you seem to be getting inconsistent measurements of the same item, that is almost always a measurement system error.
- Many processes have undocumented inspections and tests that have been developed through tribal knowledge. Check these for measurement system errors also.

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