Question

Tire Pressures. Six samples, each consisting of 10 automobile tires, were selected, and the pressure (in kilopascals, kPa) in each tire was measured. The tires were filled by a machine on an automobile assembly line. The means and ranges for the samples are shown here. Construct and analyze an ?X chart and an R chart for the data.

Sample	1	2	3	4	5	6
Mean	223.2	242	238	223	229	242
R	26.2	14.5	25.5	15.2	20.7	21.4

Answer 1

From the given data , we get

For n = 10, A₂ = 0.308, D₃ = 0.223, D₄ = 1.777

Chart

R Chart

Analysis : The process average is out of control because sample 1, 2 4 and 6 lie outside the control limits, and the process variability is in control because all the sample points fall within control limits.