1. What Are Control Limits?

Control limits are statistical boundaries used in process control that indicate the expected range of variation in a process. They are typically set at ±3 standard deviations from the process mean, which encompasses approximately 99.73% of expected variation in a normally distributed process.

2. How Does The Calculation Work?

The calculator uses the standard control limits formula:

Where:

• Mean — The average value of the process
• Sigma — The standard deviation of the process
• 3 — The number of standard deviations for 99.73% coverage

Explanation: This calculation establishes the upper and lower bounds within which a stable process should operate. Points outside these limits suggest special cause variation.

3. Importance Of Control Limits

Details: Control limits are fundamental to statistical process control (SPC) and quality management. They help distinguish between common cause variation (inherent to the process) and special cause variation (due to external factors), enabling effective process improvement and quality control.

4. Using The Calculator

Tips: Enter the process mean and standard deviation (sigma). The calculator will automatically compute the upper and lower control limits. Sigma must be a non-negative value.

5. Frequently Asked Questions (FAQ)

Q1: Why use 3 sigma instead of 2 sigma?
A: Three sigma limits (99.73% coverage) provide a better balance between detecting special causes and avoiding false alarms compared to two sigma limits (95.45% coverage).

Q2: How are control limits different from specification limits?
A: Control limits are statistically determined from process data and indicate what the process is doing. Specification limits are customer requirements that indicate what the process should do.

Q3: When should control limits be recalculated?
A: Control limits should be recalculated when the process undergoes significant changes or improvements, or when new data suggests the current limits are no longer representative.

Q4: Can control limits be used for non-normal distributions?
A: While the ±3 sigma rule is based on normal distribution, control charts can be adapted for non-normal distributions using transformations or alternative methods.

Q5: What if my data points fall outside control limits?
A: Points outside control limits indicate special cause variation that should be investigated to identify and address the root cause of the process shift.