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Standard deviation is a statistical measure that quantifies the amount of variation or dispersion in a set of data points. In control chart design, it plays a crucial role in monitoring process stability and consistency over time.
What Is Standard Deviation?
Standard deviation indicates how much individual data points differ from the average value. A low standard deviation suggests data points are close to the mean, while a high value indicates greater variability.
Importance in Control Charts
Control charts use standard deviation to establish control limits, which help determine whether a process is operating normally or if there are signs of variation that need attention. These limits are typically set at three standard deviations above and below the process mean.
Calculating Control Limits
The upper control limit (UCL) and lower control limit (LCL) are calculated using the process mean and standard deviation:
- UCL = Mean + (3 × Standard Deviation)
- LCL = Mean – (3 × Standard Deviation)
These limits help identify unusual variations that may indicate a process is out of control.