. Size of sample (n) Factor for UCL and LCL for X-Chart (A₂) Factor for LCL for R-Chart (D3) Factor for UCL for R-Chart (D4) 2 1.880 0 3.267 3 1.023 0 2.575 4 0.729 0 2.282 5 0.577 0 2.115 6 0.483 0 2.004 7 0.419 0.0076 1.924 1. At Quickie Car Wash, the wash process is advertised to take less than 7 minutes. Consequently, management has set a target average of 390 seconds for the wash process. Suppose the average range for a sample of 9 cars is 10 seconds. Use Table 3.1 to establish control limits for sample means and ranges for the car wash process.