What is a control chart in project quality management?

A control chart is a graphical display of process results over time plotted against established control limits. It is used to determine whether a process is 'in control' (within pre-defined limits) or in need of adjustment. Control charts use statistical process control to distinguish between common cause variation (normal) and special cause variation (abnormal) in a process.

What is the difference between control limits and specification limits?

Control limits represent the expected variation within three standard deviations (3σ) of the mean and reflect the 'voice of the process' — what the process is capable of producing. Specification limits reflect the 'voice of the customer' — what the customer requires. Control limits are based on actual process data, while specification limits are set by customers. Control limits are usually stricter than specification limits.

What is the Rule of Seven in control charts?

The Rule of Seven states that when seven consecutive data points fall on the same side of the mean (either all above or all below), the process is considered out of control and in need of investigation. This pattern is not considered normal variation and indicates a special cause that should be identified and addressed, even if all points are within the control limits.

Study Notes

Quality

Control Charts

Explanation of the Control Charts, Control Limits and Specification Limits in Project Quality Management