Table of Contents
What Is the Upper Control Limit?
The Upper Control Limit (UCL) is the maximum acceptable value on a control chart before a process is considered out of control. UCL is set at a specified number of standard deviations above the process mean, typically 3 sigma. Any data point above the UCL signals a potential special cause of variation that requires investigation.
Control limits are central to Statistical Process Control (SPC), developed by Walter Shewhart at Bell Labs in the 1920s. They distinguish between common cause variation (inherent to the process) and special cause variation (due to assignable factors). Points within control limits suggest the process is stable and predictable.
Formula
SPC Chart Rules
| Rule | Description |
|---|---|
| Rule 1 | Any point beyond 3σ (outside control limits) |
| Rule 2 | 9 consecutive points on same side of center |
| Rule 3 | 6 consecutive points steadily increasing or decreasing |
| Rule 4 | 14 consecutive points alternating up and down |
Frequently Asked Questions
Why use 3 sigma for control limits?
Shewhart chose 3 sigma as a practical balance. It gives a false alarm rate of only 0.27% for normally distributed data, meaning about 1 in 370 points will exceed the limits by chance when the process is in control. This minimizes unnecessary process adjustments while still detecting meaningful shifts.
Are control limits the same as specification limits?
No. Control limits are calculated from process data and reflect what the process is actually doing. Specification limits are set by the customer or designer and reflect what the process should do. A process can be in control (within control limits) but still produce items outside specification limits if the process is not capable.