Page 425 - Six Sigma Demystified

Part 3 S i x S i g m a To o l S 405

When to Use

Measure Stage
• To baseline a process by quantifying the common-cause level of variation
inherent to the process
• As part of measurement systems analysis to evaluate repeatability and
reproducibility

Analyze Stage
• To differentiate between common and special causes of variation

Improve Stage

• To verify the results of process improvement on the process metric

Control Stage

• To monitor the process to ensure the stability of the revised process and
the continued benefit of the improvement

X-bar charts are used when you can rationally collect measurements in
groups (subgroups) of more than two observations. Each subgroup represents
a “snapshot” of the process at a given point in time. The charts’ x axes are time-
based, so the charts show a history of the process. For this reason, you must
have data that are time-ordered, that is, entered in the sequence from which
they were generated. If this is not the case, then trends or shifts in the process
may not be detected but instead attributed to random (common-cause)
variation.
For subgroup sizes greater than 10, use sigma charts in place of the range
chart for monitoring process variation because the range statistic is a poor esti-
mator of process σ for large subgroups. In fact, the subgroup σ is always a better
estimate of subgroup variation than the subgroup range. The popularity of the
range chart is due only to its ease of calculation, dating to its use before the
advent of computers. For subgroup sizes equal to 1, an individual-X/moving-
range or EWMA chart can be used.
Averages are more sensitive to process shift than individual data observa-
tions. The distribution of subgroup averages for a given set of data is shown in
the top distribution curve in Figure F.56. For the same data, the distribution of