- Statistical Reference Guide
- Control charts
- Shewhart control charts
- Shewhart variables control charts
S chart
An S-chart is a type of control chart used to monitor the process variability (as the standard deviation) when measuring subgroups (n ≥ 5) at regular intervals from a process.
Each point on the chart represents the value of a subgroup standard deviation.
The center line for each subgroup is the expected value of the standard deviation statistic. Note that the center line varies when the subgroup sizes are unequal.
- A multiple (k) of sigma above and below the center line. Default k=3.
- Probability limits, defined as the probability (alpha) of a point exceeding the limits. Default alpha=0.27%.
The individual observations are assumed to be independent and normally distributed. Although the chart is fairly robust and non-normality is not a serious problem unless there is a considerable deviation from normality (Burr, 1967).
For subgroup sizes less than 6 with k-sigma control limits, the lower limit is zero. In these cases, probability limits may be more appropriate.