An x-bar and R chart is used to monitor a process. One week ago a new type of raw material was introduced and since that time 60 points have been plotted on the xbar chart and all are in the middle third of the chart. The corresponding 60 points on the R chart are all below the average range. This indicates that:

Sixty successive x-bar points confined to the centre third (≈ ±1 σ) together with sixty R-chart points below 𝑅̅ constitute strong statistical evidence that the process dispersion has fallen and the mean is now very stable. According to Western Electric/AIAG run rules, a sustained shift in spread caused by an assignable cause (here, the new raw material) requires the practitioner to freeze the chart, investigate, and—if the new condition is intended to remain—recompute 𝑋̄- and R-chart control limits from data that represent the new, reduced-variation state. Hence option C.

A. Consistent, systematic pattern across 60 observations is highly improbable from mere plotting error.
B. Gage mis-calibration would more likely inflate, not lower, the R values, or shift both charts erratically.
D. No adverse condition exists; returning to old material would discard an apparently improved process rather than establish accurate control limits.

1. Montgomery, D.C., Introduction to Statistical Quality Control, 8th ed., Wiley, 2020, §5.6, pp. 260-262 – action when runs and reduced spread occur; need to establish new limits.

2. Pyzdek, T. & Keller, P., The Six Sigma Handbook, 5th ed., McGraw-Hill/ASQ, 2022, Ch. 14, pp. 438-439 – guidance to freeze chart and recalculate limits after confirmed process change.

3. MIT OpenCourseWare 15.760 “Operations Management,” SPC lecture slides (2021), slides 19-21 – rule: prolonged points near center & reduced ranges signal new stable process; recalculate CLs.