Page 129 - Six Sigma Demystified

110 Six SigMa DemystifieD

Considering that the measurement system is used to determine acceptability
of the process relative to customer requirements, it should be apparent that as
the error increases, so does the chance that

• Products or service truly outside requirements could be measured as being
within requirements and subsequently considered acceptable.
• Products or service truly within requirements could be measured as being
outside requirements and subsequently rejected as unacceptable.
Measurement error also influences control charting and process capability
analysis. The calculation of statistical control limits and the estimation of pro-
cess capability require a sound estimate of the process variation. As measure-
ment error increases, so does the chance that
• Subgroups from a controlled process will be determined to be out of control.

• Subgroups from an out- of- control process will be determined to be from
an in- control process.

In some projects, the use of R&R studies has detected this measurement
system variability as the most significant source of common- cause variation. In
such cases, the remainder of the project addresses improvement and control of
the measurement system. Of course, the loss to the organization (before this
was determined) was quite real, so significant savings were achieved by the
project team in eliminating these measurement errors.
R&R analyses can be performed on variable or attribute (count) data. In the
case of attribute data, measurement error is often the result of the subjectivity
of the inspector, which ultimately results from requirements that have been
poorly defined. For example, if five inspectors are asked to classify a painted

sample as having a color imperfection, then, unless proper standards are estab-
lished, error often will result from the subjectivity of that assessment. How
much variation in color is acceptable? Over what distance of painted part? As
viewed under what lighting and optical conditions? An excellent discussion of
attribute gauge analysis is provided in The Six Sigma Handbook.
Examples of the use of R&R analysis may be found in Part 3.

Project examPle: Measurement Systems analysis

The process metrics are cycle time, measured in minutes, and errors, measured as
a count by error type. The precise start and end points for the cycle time measure-