The S Chart is used to observe and evaluate the variation of a process over time against control limits, and take corrective action if necessary. The chart plots the standard deviation of each of a number of sampled subgroups. It is usually plotted in conjunction with the Xbar Chart
Measuring Variation - Standard Deviation Control Chart
Reference: Statistical Process Control – Ford/GM/Chrysler
A. Sample StDev – The standard deviations of the process subgroups as collected in sequential, or chronological, order from the process
B. Sample Number – The chronological index number for the sample, or subgroup, whose standard deviation is being referenced
C. Lower Control Limit (LCL) – Line and numerical value representing the lower limit of the variation that could be expected if the process were in a state of statistical control, equal to the average Standard Deviation over the period, multiplied by a conversion factor.
D. Process Average – Overall average value of the subgroup standard deviations, over the period of inspection being referenced
E. Upper Control Limit (UCL) – Line and numerical value representing the upper limit of the variation that could be expected if the process were in a state of statistical control. It is equal to the average Standard Deviation over the period, multiplied by a second conversion factor, different from the one used to calculate the LCL.
F. Plot of the sample Standard Deviation values vs sample number.
This chart is a more accurate indicator of process variation, and is recommended for use with larger sample sizes (generally 10).
It is less sensitive than the R Chart in detecting special causes of variation that cause only a single value in a subgroup to be unusual,
n is the subgroup size, and k is the number of subgroups LCL = 0 for n<7
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