7.4 How to Read Control Charts
What is most important in process control is to grasp the state of process accurately by reading a control chart and lake appropriate actionsprom ptly when anything unusual in the process is found . The controlled state of a process is the state in which the process is stable and the process average and variation do not change. Whether aprocess is in the controlled state or not is judged by the following criteria fro m the control chart.
1) Out of control limits
Points which are outside the control limits. Run Run is the state inwhich points occur conti nually on one side of the central line and the number of points is called the length of run. Seven-point length of run is co nsidered as abnormal.
Evell if the lengthof nil'
1I11lln (, the follo w;lIg
ruses ure COli '
sidercd 10 be abnormal. a) At least 10 out of II consecutive points occur on one side of the central line. b) At least 12 out of 14consecutive points occur on one side of the central line. c) At least 16 out of 20 consecutive points occur on one side of the central line.
Seven-poi nt length of
run is abnormal.
Ten c ut orII pohns occurring on one side is abnormal.
-----'-------------Figure 7.4.1 Run
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Statistical Methods for Quality Improvement
3) Trend When the points fonn a continuousupward or downward curve, this ' is said to have a trend.
Seven upward points
Drastic down ward trend
-Figure 7.4.2 Trend
4) Approach to the control limits Considering pointswhich approach the 3-sigma control limits, if 2 out of 3 points occur outside o f the 2-sigrna lines, this case is considered to be abnormal.
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line line------- ----.--------------,
Approach 10 the Conlrol Limits (2 out 01 3 points)
5) to the central line When most of the points arc arranged within the central...