Individuals Chart

1
Individuals Chart

• Due to High Costs (e.g., destructive
  testing/measurement) or a lack of data gathering
  opportunities there may be only one measurement
  possible in a subgroup. As a rule of thumb,
  individuals charts should not be used with less
  than 100 observations.
• Control limits

2
Potential Problems With Individuals Charts

• Correlation in the moving range
• Need to be conservative in applying rules 2-5 in
  the moving range charts.
• Since the moving ranges are estimates of process
  variation, inflated control limits can result if
  the process variation is not stable throughout
  the sample.
• As a rule of thumb, if 2/3 or more of the moving
  ranges fall below the centerline, inflated
  control limits use the median of the moving range
  values instead of the average in calculating
  control limits

3

• Example (problem 7.10, page 275)