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SCHART Statement 
The following notation is used in this section:
process standard deviation (standard deviation of the population of measurements)  
s_{i}  standard deviation of measurements in i^{ th} subgroup

n_{i}  sample size of i^{ th} subgroup 
c_{4}(n)  expected value of the standard deviation of n independent normally distributed variables with unit standard deviation 
c_{5}(n)  standard error of the standard deviation of n independent observations from a normal population with unit standard deviation 
100p^{ th} percentile (0<p<1) of the distribution with n degrees of freedom 
The following table provides the formulas for the limits:
Table 40.21: Limits for s ChartsControl Limits 
LCL = lower limit 
UCL = upper limit = 
Probability Limits 
LCL = lower limit 
UCL = upper limit 
The formulas assume that the data are normally distributed. If a standard value is available for ,replace with in Table 40.21. Note that the upper and lower limits vary with n_{i} and that the probability limits are asymmetric around the central line.
You can specify parameters for the limits as follows:
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