The X-Individual chart is sometimes called a pre-control chart. This type of chart is used to determine the stability of the center of the process. The control limits are calculated by determining the standard deviation of the sample population.
Center Line
For the X-Individual chart, the center line is calculated as:
where x1 through xn are the values of each item in the set and n is the number of samples.
Standard Deviation
The standard deviation is calculated as:
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S = |
MR2 |
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1.128 |
Where MR2 = the Mean Moving Range of length 2 (Average of Ranges taken with moving length of 2)
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MR2 = |
(SUM(|Xi - Xi-1|)) |
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n-1 |
Where i > 1
Control Limits
The formulas used for calculating the upper and lower control limits are:
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Upper Control Limit |
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Lower Control Limit |
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XBar-R, XBar-S, R, S and X chart calculations. |
