Page 314 - The Handbook for Quality Management a Complete Guide to Operational Excellence

300 C o n t i n u o u s I m p r o v e m e n t M e a s u r e S t a g e 301

Definitions
Bias. The difference between the average measured value and a reference
value is referred to as bias. The reference value is an agreed-upon stan-
dard, such as a standard traceable to a national standards body. When
applied to attribute inspection, bias refers to the abili ty of the attribute
inspection system to produce agreement on inspec tion standards. Bias is
controlled by calibration, which is the process of comparing measurements
with standards. The concept of bias is illus trated in Fig. 14.2.

Repeatability. AIAG defines repeatability as the variation in measurements
obtained with one measurement instrument when used several times
by one appraiser, while measuring the identical characteristic on the
same part. Variation obtained when the measurement system is applied
repeatedly under the same conditions is usually caused by conditions
inherent in the measurement system. ASQ defines precision as
“the closeness of agreement between randomly selected individual
measurements or test results. NOTE: The standard deviation of the error
of measurement is sometimes called ‘imprecision.’ ” This is similar to
what we are calling repeatability. Repeatability is illustrated in Fig. 14.3.
Reproducibility. Reproducibility is the variation in the average of the
measurements made by different appraisers using the same measuring
instrument when measuring the identical characteristic on the same
part. Reproducibility is illustrated in Fig. 14.4.
Stability. Stability is the total variation in the measurements obtained
with a measurement system on the same master or parts when

Figure 14.2 Bias Bias
illustrated.

Reference Average
value measurement

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