What is a basic control chart used in quality control and what does it monitor?

What is a basic control chart used in quality control and what does it monitor?

• A. P-chart monitors defect rate.
• B. An X-bar chart monitors the average of samples over time to detect shifts in the process mean.
• C. S-chart monitors standard deviation of a sample.
• D. Individuals chart monitors single measurements.

Answer: (B)

In quality control, a basic approach for tracking what the process is doing over time is to monitor the average of small samples. The X-bar chart is built by taking measurements in subgroups, computing the average of each subgroup, and plotting those averages over time. This directly shows whether the process mean is stable or shifting. Control limits around the average indicate expected natural variation; if a subgroup mean falls outside these limits or displays a nonrandom pattern, it signals a potential shift in the process mean that should be investigated. Choosing other charts wouldn’t fit the focus here: a chart of defect rates (proportions) looks at attributes of output rather than measurement means, a chart of standard deviations looks at variability rather than the average, and a chart of individual measurements uses single data points instead of subgroup averages.

In quality control, a basic approach for tracking what the process is doing over time is to monitor the average of small samples. The X-bar chart is built by taking measurements in subgroups, computing the average of each subgroup, and plotting those averages over time. This directly shows whether the process mean is stable or shifting. Control limits around the average indicate expected natural variation; if a subgroup mean falls outside these limits or displays a nonrandom pattern, it signals a potential shift in the process mean that should be investigated.

Choosing other charts wouldn’t fit the focus here: a chart of defect rates (proportions) looks at attributes of output rather than measurement means, a chart of standard deviations looks at variability rather than the average, and a chart of individual measurements uses single data points instead of subgroup averages.