666
LCL = 4.89
CEN = 17.278
0 5 101520253035
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Moving Range Chart for 2000, 2001, 2002
UCL = 15.215
LCL = 0.0
CEN = 4.657
??“4
??“2
0
2
4
6
8
1012141618
2000
2001
2002
Figu
re 18.13
Xmr chart for 20
00, 2001, and 2002.
Statistical Process Control n 253
and June. The charts do not tell us why, but we can clearly see that something is happening.
We need to investigate that something to determine what is going on.
Computing limits for the Xmr Chart
We know that the values for the center lines for each chart were computed by
simply taking the average of the values displayed (that is, by adding up the values
for each month and then dividing by the number of months/values to compute the
average). How were the upper and lower limits for the charts shown calculated? We
can calculate the limits for both the X (individual values) chart and the average
moving range (mR) chart as follows:
For the mR (moving range) chart.
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