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The Use of Six Sigma with High- and Low-Volume Products and Processes








                                    Figure 5.6 Sources of process variation and error.  155


                          Precision is the relative amount of the variability of the measuring
                          system; hence, it is an indicator of the variability of the equipment
                          (gauges).
                          Accuracy is a relative measure of achieving the measurement tar-
                          get. It is the difference between the true and measured values, al-
                          though the true value may not be known in many cases. Accuracy
                          is  usually  referred  according  to  some  standard  of  measurement.
                          Hence, it is an indicator of the measurement error average of   of
                          the equipment (gauges). Accuracy and precision are shown in Fig-
                          ure 5.7, using a target analogy.
                          Repeatability  is  a  measure  of  the  consistency  of  readings  of  the
                          same part for a single operator (appraiser). Poor repeatability indi-
                          cates  measurement  system  problems  related  to  equipment.  Re-
                          peatability is derived from the same operator measuring different
                          parts repeatedly using the same measurement equipment. Some-
                          times it is called precision or equipment variation (EV).
                          Reproducibility is a measure of variation in average measurements
                          when  different  operators  are  taking  many  measurements  of  the
                          same part. It can be used as measure of the relative amount of train-
                          ing or skills for the operators. Sometimes it is called appraiser vari-
                          ation (AV), using the same parts and gauges and different operators.
                          GR&R is the root sum of the squares (RSS) value of repeatability
                          and  reproducibility.  It  should  be  noted  that  the  average  or  the
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