Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 517 of 707

control, analysis, and testing UNDERSTANDING ACCURACY, REPEATABILITY, AND REPRODUCIBILITY BY FRANCIS REILLY MATRIX METROLOGIES, HOLBROOK, NY; Accuracy, repeatability, and reproducibility are three terms that are used exten- sively yet are poorly understood. In manufacturing a product, the producer must be responsible for delivering accurate output in accordance with a given spec- ification. This must be performed to within a certain level of tolerance—precision. It must be able to be duplicated—reproducible. In order for a process to consistently output accurate, precise, and reproducible product, these parameters must be understood and monitored (metrics). In general, manufacturers will use some collection of analytical tools and gauges to gather the metrics required to perform this task. Each of these devices will measure some parameter of the process to determine that specific process vari- ables' accuracy, repeatability, and reproducibility attributes. Understanding this information allows subsequent process improvement. The following definitions describe accuracy, repeatability, and precision with- in the context of measuring variations within a process: • Accuracy: Refers to how closely a measurement value obtained by an instrument conforms to the actual ("absolute") value of the sample. • Repeatability: The ability of an instrument to produce consistent results when performing multiple measurements on the same sample area. Also referred to as precision. • Reproducibility: The ability of an instrument to obtain consistent measurement results when measuring the same sample at different times, or made by different operators, or by using different instruments of the same type. ILLUSTRATING THE DIFFERENCES The classical bull's eye chart is a great way to envision and remember the differ- ences between these concepts. In the Figures 1 and 2 the x represents the output of a specific process, such as the plating thickness of a part. Successive x's are addi- tional parts coming off of the line. The goal of this process is to be accurate (in the middle of the range) and to be able to be accurate again and again (precise). Random error represents the inability to be consistent. Sometimes you hit the target and sometimes you don't. Systemic error, on the other hand, represents a consistent problem or bias in the process. You are repeatable but you are not accu- rate. For example, you consistently plate too thick. REPRODUCIBILITY Reproducibility is perhaps the most misunderstood concept of the three. It is effec- tively the ability to reproduce results at a different time or place. Figure 2 shows both a highly reproducible and and a non-reproducible process. In this example, each measurement set is the exact same set of measurements repeated at future 516

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