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Twenty samples of subgroup size 6 (n = 7) were collected for a variable measurement. Determine the upper control limit (UCL) for an R-chart if the mean of the sample ranges equals 6.4.
 
Factors for calculating control limits
n                 A2      D3             D4
2              1.880     0              3.267
3              1.023     0              2.574
4              0.729     0              2.282
5              0.577     0              2.114
6              0.483     0              2.004
7              0.419     0              1.924

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