Which item is plotted over time in a control chart to indicate process stability?

Which item is plotted over time in a control chart to indicate process stability?

• A. A statistic such as the process mean or range
• B. A histogram of defects
• C. A scatter plot of two variables
• D. A bar chart of categories

Answer: (A)

A control chart shows how a process characteristic behaves over time by plotting a statistic calculated from samples, such as the process mean (X-bar) or the range. By placing these statistics on a time sequence with control limits, you can tell if the process is stable (points stay within limits and patterns are random) or if special causes are present (points outside limits or nonrandom trends). The best choice is a statistic like the process mean or range because that is precisely what a control chart tracks over time to assess stability. The other visualizations aren’t designed to monitor a process characteristic through time: a histogram displays defect frequency distribution, a scatter plot shows relationships between two variables, and a bar chart compares categories, none of which track process stability over time.

A control chart shows how a process characteristic behaves over time by plotting a statistic calculated from samples, such as the process mean (X-bar) or the range. By placing these statistics on a time sequence with control limits, you can tell if the process is stable (points stay within limits and patterns are random) or if special causes are present (points outside limits or nonrandom trends). The best choice is a statistic like the process mean or range because that is precisely what a control chart tracks over time to assess stability. The other visualizations aren’t designed to monitor a process characteristic through time: a histogram displays defect frequency distribution, a scatter plot shows relationships between two variables, and a bar chart compares categories, none of which track process stability over time.