Which diagram is used to graph process performance data against predetermined control limits to detect assignable causes of variation?

• A. Pareto analysis
• B. Control chart
• C. Process capability index
• D. Scatter plot

Answer: (B)

A control chart plots process data over time and compares it to predetermined control limits to detect assignable causes of variation. The center line represents the process average, with upper and lower control limits showing the range of typical, common-cause variation. If data stay within these limits and show no nonrandom patterns, the process is considered in control. When a point falls outside the limits or patterns emerge (such as a trend or a run of points), it signals a special, assignable cause that should be investigated. This makes the diagram ideal for ongoing monitoring of performance and quickly detecting when something unusual is affecting quality. Other diagrams serve different purposes: Pareto analysis ranks causes by frequency or impact but doesn’t plot performance against control limits; a scatter plot shows relationships between two variables; a process capability index assesses how well the process meets specifications rather than monitoring for assignable causes.