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FEATURE STORY:
R&R studies support measurement
systems reliability

To those unfamiliar with measurement systems analysis, “R&R” may sound like a recreational program. To others, it may seem to be an arcane and sophisticated statistical technique—something to be avoided at all cost.

R&R—repeatability and reproducibility—has nothing to do with a respite on a tropical island, nor with highly complicated statistical techniques. Instead, it offers an approach to measurement systems that verifies their reliability and enhances their accuracy. R&R will check for bias among appraisers; ideally, there will be no bias, and the reproducibility component of R&R should be zero. Undertaking an R&R study involves considering a number of factors, and when these factors are taken into consideration, the process offers a simple solution to determining reliability of measurement systems.

The objective of a simple R&R study is to determine whether the measurement system in use can adequately distinguish between or among units. A number of factors affect the ability of a measurement system to discriminate among the units it measures. These factors may be categorized as those that typically contribute to any process variation: machine (the gage), operator (appraiser), method (method of measurement followed), material (units being measured), and environment. 

In conducting a simple study, an attempt is made to minimize or hold most of these factors constant. (Note: The study should replicate as much as possible the conditions under which the process measurement is occurring. If production operators are making the measurements, the study should use production operators under the same conditions that exist when they are “normally” measuring the output. The study should not be conducted using quality technicians in a lab.) Therefore:

  1. One gage is used (no inter-gage variation).
  2. The same method of measurement is to be employed by all the appraisers involved (no variation due to appraisers’ use of different methods).
  3. The same dimension on each part is measured each time. A further assumption is that each part is measured in the same place, to eliminate the possibility of within-part variation (no variation introduced due to different characteristics or measurement taken at a different location on the part).
  4. The study is conducted under the same conditions that exist when the parts are normally measured (no variation introduced by the location or environmental concerns related to where the measurements are taken).

These steps are expanded and explained in a white paper produced by R&R expert Gordon K. Constable, Ph.D. Download a free copy of this paper.

 
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