What does the 3 Sigma Limit indicate in a control chart?

The 3 Sigma Limit in a control chart is a critical statistical concept used to determine the control limits for a process. It signifies the threshold at which a process is statistically significant from its average performance due to natural variability, indicating that the process may be out of control.

When data points from a process fall outside the 3 Sigma limits (which are typically set at three standard deviations from the process mean), it suggests that there is a higher probability that special causes of variation are influencing the process. This allows analysts and quality professionals to detect anomalies or trends that could indicate problems needing investigation. The primary purpose of these limits is to help maintain the stability of the process, ensuring it operates efficiently and produces quality outputs.

The other choices relate to different aspects of quality management but don’t accurately describe the function of the 3 Sigma Limit. The first option refers to defect thresholds, the third option speaks to average performance levels, and the fourth option implies a baseline quality measure. None of these capture the essential role of the 3 Sigma Limit in monitoring process control.