Pre-control charts are a simple form of process monitoring used primarily for short production runs or setups.
Unlike traditional control charts, the limits of pre-control charts are not based on statistical calculations of process variation but are derived directly from specification limits.
* Specifications:
* Pre-control chart limits are determined by dividing the specification range into zones.
* Typically, the specification limits are divided into green, yellow, and red zones to indicate areas of acceptable and unacceptable performance.
* The green zone is typically the middle 50% of the specification range, yellow zones are the next
25% on either side, and the red zones are the outer 25%.
* Natural Tolerance:
* Refers to the spread of the process data within ±3 standard deviations from the process mean, but not used in pre-control charts.
* Process Standard Deviation:
* Used in traditional control charts to calculate control limits, not in pre-control charts.
* Process Capability:
* Evaluates how well a process can meet specifications, but pre-control limits are directly based on the specifications themselves.
References: Pre-control chart methodology and usage guidelines.