When X-bar and R charts are used, what do these charts monitor?

When X-bar and R charts are used, what do these charts monitor?

• A. X-bar monitors the range; R monitors the mean.
• B. X-bar monitors the mean; R monitors the range of the subgroup.
• C. X-bar monitors the mean; R monitors the range of the entire process.
• D. X-bar monitors the variance; R monitors the median.

Answer: (C)

These charts are used together to monitor both the center and the spread of data collected in subgroups over time. For each subgroup, you compute the average and the range. The X-bar chart plots the subgroup means, showing whether the process average is stable or drifting. The R chart plots the subgroup ranges, showing how much variation exists within each subgroup. The key idea is that the mean reflects the process center, while the range captures dispersion within subgroups. The range being tracked is specifically the variation inside each subgroup, not the entire process across all data. That’s why X-bar monitors the mean and R monitors the range of the subgroup. If both charts stay in control, the process is considered stable; if either chart shows a signal, investigate the underlying causes affecting mean or dispersion.

These charts are used together to monitor both the center and the spread of data collected in subgroups over time. For each subgroup, you compute the average and the range. The X-bar chart plots the subgroup means, showing whether the process average is stable or drifting. The R chart plots the subgroup ranges, showing how much variation exists within each subgroup. The key idea is that the mean reflects the process center, while the range captures dispersion within subgroups. The range being tracked is specifically the variation inside each subgroup, not the entire process across all data. That’s why X-bar monitors the mean and R monitors the range of the subgroup. If both charts stay in control, the process is considered stable; if either chart shows a signal, investigate the underlying causes affecting mean or dispersion.