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IRCHART Statement 
The following notation is used in this section:
process mean (expected value of the population of measurements)  
process standard deviation (standard deviation of the population of measurements)  
X_{i}  the i^{ th} individual measurement 
mean of the individual measurements, computed as (X_{1}+ ... +X_{N})/N, where N is the number of individual measurements  
n  number of consecutive measurements used to calculate the moving ranges (by default, n=2) 
R_{i}  moving range computed for the i^{ th} subgroup
(corresponding to the i^{ th} individual measurement).
If i<n, then R_{i} is assigned a missing value.
Otherwise,

average of the nonmissing moving ranges, computed as
 
d_{2}(n)  expected value of the range of n independent normally distributed variables with unit standard deviation 
d_{3}(n)  standard error of the range of n independent observations from a normal population with unit standard deviation 
z_{p}  100p^{ th} percentile (0<p<1) of the standard normal distribution 
D_{p}(n)  100p^{ th} percentile (0<p<1) of the distribution of the range of n independent observations from a normal population with unit standard deviation 
Each point on a moving range chart indicates the value of a moving range (R_{i}). With n=2, for example, if the first three measurements are 3.4, 3.7, and 3.6, the first moving range is missing, the second moving range is 3.73.4=0.3, and the third moving range is 3.63.7=0.1.
The central line on a moving range chart indicates an estimate for the expected moving range, computed as where .If you specify a known value () for , the central line indicates the value of .
The following table provides the formulas for the limits:
Table 34.22: Limits for Individual Measurements and Moving Range ChartsControl Limits  
Individual Measurements Chart  LCL = lower control limit 
UCL = upper control limit  
Moving Range Chart  LCL = lower control limit 
UCL = upper control limit = 
Probability Limits  
Individual Measurements Chart  LCL = lower control limit 
UCL = upper control limit  
Moving Range Chart  LCL = lower control limit 
UCL = upper control limit 
The formulas assume that the measurements are normally distributed. Note that the probability limits for the moving range are asymmetric about the central line. If standard values and are available for and ,replace with and with in Table 34.22.
You can specify parameters for the limits as follows:
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