Which chart is used to monitor the variability within subgroups, rather than their means?

• A. X-bar chart.
• B. P-chart.
• C. Histogram.
• D. S chart.

Answer: (D)

Monitoring variability inside subgroups is done by charts that plot dispersion rather than the average. An S chart calculates the standard deviation within each subgroup and plots those values over time, revealing whether the spread of data within subgroups stays stable. This focuses on how much the data vary, not on the subgroup means themselves. In contrast, charts that track means, like the X-bar chart, look at central tendency; a P-chart monitors the proportion defective, and a histogram shows the overall distribution rather than subgroup dispersion over time. Therefore, the chart that monitors variability within subgroups is the S chart.