Up to three standard deviations above or below the centerline is the amount of variation that statistical process control allows for:

Statistical process control (SPC) is a data-driven process that uses statistical methods to monitor and control the quality of a production process. The goal of SPC is to ensure that the process is stable, consistent with customer requirements, and not drifting or changing in an unexplainable manner. 

SPC is commonly used in manufacturing or production processes to measure how consistently a product performs according to its design specifications. Some of th