Which chart is used in Statistical Process Control to detect assignable causes?

• A. Pareto chart.
• B. Control charts.
• C. Box plot.
• D. Scatter plot.

Answer: (B)

In Statistical Process Control, you monitor a process over time to spot when something unusual has happened. Control charts are the tool designed for this: they plot measurements in order of collection and mark upper and lower control limits based on the process’s expected (in-control) variation. If data stay within these limits and show no nonrandom patterns, the variation is considered normal. But when a point falls outside the limits or a recognizable pattern appears (like a run, trend, or cycles), that signals an assignable cause—a specific issue that can be investigated and corrected. This ability to flag those specific disturbances is what makes control charts the go-to method for detecting assignable causes.

The other charts have different purposes. A Pareto chart helps prioritize problems by frequency but doesn’t monitor a process over time for assignable disturbances. A box plot summarizes the distribution of a single dataset without considering time or process signals. A scatter plot shows the relationship between two variables, not the ongoing process behavior needed to detect assignable causes.