The upper and lower limits of a control chart are calculated from actual process measurements. These limits, often set at ±3 standard deviations from the process mean, represent the expected range of variation in the process due to common causes. Data points outside these limits may indicate the presence of special cause variation, signaling that the process is not in control and requires investigation.
* Calculated by projecting future requirements (B): Control limits are based on current process performance, not future projections.
* Derived from special cause variation (C): Control limits are established to identify special cause variation, not derived from it.
* Derived from external regulatory standards (D): While external standards may influence quality goals, control limits are based on internal process data.
References
* NAHQ Body of Knowledge: Process Control and Control Charts
* NAHQ CPHQ Exam Preparation Materials: Understanding Control Limits and Process Variation
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