The limits of a pre-control chart are based on

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.