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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