Page 364 - Six Sigma Demystified

344 Six SigMa DemystifieD

Interpretation

Discrimination Ratio
The discrimination ratio provides an estimate of the number of distinct catego-
ries or classes discernible by a measurement system. A value of 2, for instance,
indicates that the system is useful only for attribute analysis because it can ef-
fectively classify the sample units only as being in one group or the other (e.g.,
good or bad). AIAG recommends a value of 3 or more, and typically a value of
8 or more is suitable for statistical analysis.

Gauge R&R
The gauge R&R statistic is usually reported as either a percent of tolerance or
as a percent of the total process variation. When expressed as percent of toler-
ance, gauge R&R indicates the relative usefulness of the gauge system for de-
termining acceptability of the process output. Tolerance, calculated as the
upper specification limit minus the lower specification limit, indicates the
amount of variation that is permitted. As measurement system error increases
relative to tolerance, the chances increase that

• Output whose true value is beyond specifications could be measured as
being within specifications and subsequently considered acceptable.
• Output whose true value is within specifications could be measured as
being outside specifications and subsequently rejected as unacceptable.

When expressed as a percent of process variation, R&R indicates the relative
usefulness of the measurement system in control charting and process capabil-

ity analysis. The calculation of statistical control limits and the estimate of
process capability require a sound estimate of process variation. Subgroups
plotted on a control chart are subject to this measurement system error as well.
As measurement error increases, so does the chance that

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

Typical recommendations for R&R, expressed as either percent of tolerance
or percent of process variation, are