If a process has Outliers which pair of charts is most preferable if subgroups will exist for the Continuous Data?

Question

Options

• AIndividual-Moving Range
• BXbar-R Charts
• CXbar-S Charts
• DnP and P Charts

• A
  6% (1)
• B
  71% (12)
• C
  6% (1)
• D
  18% (3)

Explanation

Xbar-R Charts (B) are the correct choice because the Range (R) statistic is more robust to outliers than the Standard Deviation (S) statistic - outliers inflate S dramatically (due to squaring of deviations), making Xbar-S unreliable when outliers are present, while the range limits their distorting effect. Both Xbar charts require subgroups to exist, which the question confirms, making them appropriate for this scenario.

Why distractors are wrong:

• A (Individual-Moving Range): Used when no subgroups exist (subgroup size = 1); the question explicitly states subgroups will be present.
• C (Xbar-S): Also requires subgroups and handles continuous data, but the S chart uses standard deviation, which is highly sensitive to outliers - making it the wrong choice when outliers are a concern.
• D (nP and P Charts): These are attribute charts used for proportion/count of defective units (discrete data), not continuous measurements.

Memory tip: Think "R for Resistant" - when outliers threaten your process data, reach for the Range-based chart. Also remember the elimination order: outliers → avoid S; no subgroups → avoid Xbar charts; discrete data → avoid Xbar altogether.

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