Which tool would you use to determine if a process is in statistical control?

Which tool would you use to determine if a process is in statistical control?

• A. Control chart
• B. Scatter diagram
• C. Pareto chart
• D. Fishbone diagram

Answer: (A)

Determining whether a process is statistically in control comes down to evaluating how its output behaves over time and whether the variation seen is just the normal, inherent variation of the process or due to identifiable, assignable causes. A control chart is built specifically for this purpose: it plots measurements in time with a centerline (the process average) and upper and lower control limits. When the process is in control, data points wander randomly around the center within the limits and no nonrandom patterns appear. If a point falls outside the limits or a recognizable pattern shows up—such as a persistent trend, a run of points on one side, or cycles—that signals an assignable cause and that the process may be out of control, prompting investigation and corrective action. The other tools have different roles: a scatter diagram shows the relationship between two variables, a Pareto chart highlights the most frequent problems, and a fishbone diagram helps identify potential root causes. None of these directly indicate whether the process is in statistical control over time.

Determining whether a process is statistically in control comes down to evaluating how its output behaves over time and whether the variation seen is just the normal, inherent variation of the process or due to identifiable, assignable causes. A control chart is built specifically for this purpose: it plots measurements in time with a centerline (the process average) and upper and lower control limits. When the process is in control, data points wander randomly around the center within the limits and no nonrandom patterns appear. If a point falls outside the limits or a recognizable pattern shows up—such as a persistent trend, a run of points on one side, or cycles—that signals an assignable cause and that the process may be out of control, prompting investigation and corrective action.

The other tools have different roles: a scatter diagram shows the relationship between two variables, a Pareto chart highlights the most frequent problems, and a fishbone diagram helps identify potential root causes. None of these directly indicate whether the process is in statistical control over time.