Use Minitab to construct an X and R-chart from this data set. Is the process stable? If no, at what times is it unstable? Answer: No, when 9:30am, 10:15am, 10:30am, 3:15pm and 4:00pm.

Question

Options

• ANo, when 9:30am, 10:15am, 10:30am, 3:15pm and 4:00pm.

• A
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Explanation

Option A is correct because when constructing X-bar and R-charts in Minitab, a process is deemed unstable (out of statistical control) whenever one or more data points fall outside the upper or lower control limits (UCL/LCL), or when non-random patterns violate Western Electric rules. The Minitab output flags the subgroups at 9:30am, 10:15am, 10:30am, 3:15pm, and 4:00pm as rule violations - these points signal assignable (special) causes of variation rather than natural process noise.

Since only one choice is presented here, there are no distractors to refute; the question is testing whether you can correctly read Minitab's control chart output and identify which specific time points are marked as out-of-control signals.

Memory tip: Think of control chart violations as "alarm bells" - Minitab literally flags the offending points. When asked if a process is stable, scan both charts (X-bar for mean shifts, R-chart for variability spikes). If any point is flagged on either chart, the answer is "No", and you list exactly those flagged time points. Clustering of flags (e.g., 10:15 and 10:30 are back-to-back) often hints at a sustained disturbance like a machine adjustment or operator change around that time.

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