Length is a stable and normal process variable that has been monitored for several weeks. Analysis confirms that the length sample averages (i) is 3.66 mm and fewer than 10% of individual measurements are within 1 0.28 mm of the mean. Control charts of n units are to be established for this process. When n = 7, the i lower control limit ( LCL } value for an i-R chart is 3.41 mm