Control charts for X and R are to be established on a certain dimension part, measured in millimeters. Data were collected in subgroup sizes of 6 and are given below. Determine the trial central line and control limits.

Assume assignable causes and revise the central line and limits.

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Subgroup Number X R Subgroup Number X R 1234567890122 20.35 0.34 14 20.41 0.36 20.40 0.36 15 20.45 0.34 20.36 0.32 16 20.34 0.36 20.65 0.36 17 20.36 0.37 20.20 0.36 18 20.42 0.73 20.40 0.35 19 20.50 0.38 20.43 0.31 20 20.31 0.35 20.37 0.34 21 20.39 0.38 20.48 0.30 22 20.39 0.33 20.42 0.37 23 20.40 0.32 20.39 0.29 24 20.41 0.34 20.38 0.30 25 20.40 0.30 20.40 0.33