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About this lesson
Gage R&R Principles
Gage R&R Studies are used to determine if the common cause variation that effects measurement system precision is acceptable. This lesson introduces Gage R&R Studies and explains the principles used in conducting them.
When to use
A Gage R&R Study should be done on a measurement system when it is first put into service to qualify it. Also, a study should be done after a major change to the system or prior to use in a new or unusual manner. Studies are also done periodically for those systems that are deemed to be vulnerable to high variation.
Gage Repeatability and Reproducibility (R&R) Studies are designed to measure the common cause variation of a measurement system. It quantifies the level of measurement error that affects the precision of the system. Gage R&R Studies are often done as part of the qualification of a new measurement system. They are also done after a major change to the system or the system’s procedures. If the system is to be used in a new or unique manner, such as to conduct special tests for a Lean Six Sigma project, a Gage R&R Study is often done to ensure the data will be meaningful.
Some organizations periodically do a Gage R&R study if the measurement system is susceptible to many factors that can cause variation. This is in order to monitor the performance of the system. A Gage R&R Study includes equipment, associates who normally use the equipment, items for inspection, and other factors that will usually be elements of the measurement process. The key is to conduct the measurements in a manner that is as normal as possible to minimize the introduction of special causes. Several typical characteristics of a study are:
- Two or three appraisers who are individuals who normally use the measurement system for the application being studied.
- Multiple items for inspection. Use five to ten items for variable measurements and twenty to thirty for attribute measurements.
- Items for measurement should be the full range of typical values and include some “bad” items that are outside the normal range of acceptability.
- Each item is measured by each appraiser multiple times – normally two or three trials.
- Items are measured in a random order.
- Items are indistinguishable from each other by the appraisers.
- Appraisers use the same measurement system equipment.
- Measurements are done in the actual work location if possible. If not, the setting is as similar as possible.
- Data is carefully collected so that each data reading is correlated to a specific appraiser, a specific item and a specific trial – although the appraiser cannot know which item is which.
The results of the data are statistically analyzed and repeatability issues, which is variation that an operator has with the system, and reproducibility issues, which is variation between operators, is quantified.
Hints & tips
- The person conducting the study is not normally one of the appraisers. That way they can manage the data collection form and ensure data is recorded correctly.
- Use Minitab to create the data collection form. It will randomize the tests and is an easy way to store the data. Plus, it is ready for analysis as soon as the data collection is done.
- The biggest danger when doing these is that the appraisers know it is a special study, so they change their normal measurement habits or techniques. This introduces a special cause and can invalidate the study findings. That is why we try to do these in the normal workplace. If it is a familiar environment they are less likely to change behavior.
- The study must be balanced – all appraisers measure all items the same number of times – for the statistics to work correctly.
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