Publication date: 08/13/2020

Control Limits for XBar and R Charts

JMP generates control limits for XBar and R charts as:

LCL for XBar chart =

UCL for XBar chart =

LCL for R chart =

UCL for R chart =

Center line for R chart: By default, the center line for the ith subgroup (where 3 is the sigma multiplier) indicates an estimate of the expected value of Ri. This value is computed as: , where is an estimate of σ.

The standard deviation for XBar and R charts is estimated by:

where:

= weighted average of subgroup means

σ = process standard deviation

ni = sample size of ith subgroup

d2(n) is the expected value of the range of n independent normally distributed variables with unit standard deviation

d3(n) is the standard deviation of the range of n independent normally distributed variables with unit standard deviation

Ri is the range of ith subgroup

N is the number of subgroups for which ni ≥ 2.

Want more information? Have questions? Get answers in the JMP User Community (community.jmp.com).
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