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Control Limits Formula:
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Control limits are statistical boundaries used in quality control processes to determine if a process is in a state of statistical control. The upper control limit (UL) and lower control limit (LL) are typically set at ±3 standard deviations from the process mean.
The calculator uses the control limits formula:
Where:
Explanation: These limits define the range within which we expect the process to operate when it is in statistical control. Points outside these limits indicate special cause variation.
Details: Control limits are essential for statistical process control (SPC) and help identify when a process is experiencing unusual variation that requires investigation and corrective action.
Tips: Enter the process mean and standard deviation. The calculator will compute the upper and lower control limits at ±3 standard deviations from the mean.
Q1: Why use 3 standard deviations for control limits?
A: Three standard deviations capture approximately 99.73% of the data in a normal distribution, providing a balance between sensitivity to special causes and false alarms.
Q2: How are control limits different from specification limits?
A: Control limits are based on process data and variability, while specification limits are defined by customer requirements or design specifications.
Q3: When should control limits be recalculated?
A: Control limits should be recalculated when there are significant process changes, improvements, or when the process demonstrates sustained stability over time.
Q4: Can control limits be used for non-normal distributions?
A: While traditionally used with normal distributions, control limits can be adapted for non-normal distributions using appropriate transformations or alternative methods.
Q5: What does it mean if data points fall outside control limits?
A: Points outside control limits indicate special cause variation that requires investigation to identify and eliminate the root cause.