Reading a control chart

1. HOW TO READ A
CONTROL CHART
P R E PA R E D BY ; C H E M E N G I C H S A M M Y J A M A R
P R E S E N T E D TO ; P R O F. M O H A M M A D Y O U N E S 1

2. CONTROL CHART
• The Western Electric rules are decision rules in statistical process control
for detecting out-of-control or non-random conditions on control
charts. Locations of the observations relative to the control chart control
limits (typically at ±3 standard deviations) and centerline indicate
whether the process in question should be investigated for assignable
causes.
• We use the western electric rules to read control charts and determine
their conformance to the required measurements and adjust accordingly
when out of control.
• They are applied as follows;
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3. RULE 1
• A process is assumed to be out of control if a single point plots outside
the control limits. This is the most commonly used rule. For a process in
control, if control limits are three standard deviations from the mean of
the characteristic being plotted, the chance of this happening is quite
small (only 0.0026), assuming normality.
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4. RULE 2
• A process is assumed to be out of control if two out of three
consecutive points fall outside the two-sigma limits on the same side of
the center line. The two-sigma limits are those that are two standard
deviations from the mean of the quality characteristic.
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5. RULE 3
• A process is assumed to be out of control if four out of five
consecutive points fall beyond the one-sigma limit on the
same side of the center line.
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6. RULE 4
• A process is assumed to be out of control if nine or more
consecutive points fall to one side of the center line.
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7. END.
THANK YOU.
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